Get some Magnesium Citrate. Take 200-600 mg before bed. Many low-carbing Paleos are deficient and it will really help with constipation.

Google FODMAPS. You may be having trouble digesting all those vegetables if your gut flora is out of whack.

Drop the Dr. Pepper!

Diet Dr. Pepper would be my primary concern.

Excessive bacon/sausage - unless sourced well these can be/are usually full of some fairly nasty chemicals and rancid fats.

Fish oil supplements normally aren't needed if you are sourcing good meat, and bad fish oil supplements (shelf-stabilized enteric capsules) can cause some of the problems you are experiencing (at least they did for me).

So that's what I think. Have you tried simplifying the foods you are eating to the point of "only whole foods", i.e. nothing "with an ingredients label" to see if the issues get better/worse? Replace some of that bacon/sausage with real meat and just drink water. Perhaps skip the fish oil as well... (in fact, the more I think about it, I think the DDP and Fish Oil are both fairly detrimental to healing upper GI issues).

First time I've felt compelled to respond on these boards. Here we go...

About a month ago a relative began experiencing a weird pressure in her chest, felt a frequent need to belch, a sense of fullness that made eating difficult sometimes, an upset stomach, fatigue, shortness of breath, etc.

They did an ultrasound a week later and noticed liver cysts and a fatty liver. Supposedly the cysts are what's causing the problems. Just a suggestion, but you may want to get your gallbladder and liver checked. They said the cysts were pushing up and causing the tightness, shortness of breath, etc. This relative is an RT and was totally surprised by the connection to breathing trouble.

GL and I hope you get to feeling better soon.

I did "paleo" for a minute before kicking my diet coke habit. I don't count my true adoption of the paleo lifestyle until I actually kicked the diet coke habit. Diet soda pretty much negates many of the intangible "feel better" benefits of paleo, and from your post, it's pretty clear to me that this is your problem.

Agree with all of the above. Carbonation from the soda, fish oil burps, too much sugar fermenting in your gut (FODMAP's) can all cause your stomach distress. Perhaps you wouldn't have had any trouble if it was just one thing but all together it's acting synergistically.

"Also eating lots of rotisserie chicken." Home cooked or store bought? Supermarket purchased rotisserie chicken causes issues for me. Flavour enhancers or other additives may be used. I am in Australia so this may not apply to you. Adding this to the preservatives/chemicals that may be in bacon and sausages and diet soda doesn't sound too healthy to me.

I don't know if it is just the diet soda for you, but I can tell you I gave them up pretty fast as I went paleo. They made my stomach feel like it was going to pop. I had felt just fine drinking them before.

I stopped drinking all diet soda when I received a diagnosis of leukemia. Aspartame...not so good for you. But even if its not health your worried about, the carbonation could be more than your body wants to handle while making the switch to paleo. Give it up for a few days and see what happens. I would give up the diet soda for a trial period before giving up eating right.

Certain veg (e.g. Romaine and iceberg lettuces, broccoli, raw cabbage) makes me feel ridiculously full for hours and hours. I have found relief by drinking 2 tablespoons of unfiltered apple cider vinegar.

Also, like others have said, dump the DDP. The chemicals in it are no good!

At the risk of sounding like a parrot, "drop the soda! drop the soda!".

Having addressed the obvious, I have to ask -- how much fat are you eating?

Because what you are describing (abdominal distention and postural pain, disruptions in bowel routine) sounds an awful lot like....

[drumroll]

gallbladder trouble! :)

Monarchs living high off the hog used to get this. In France they call it crise de foie ("liver crisis") and the standard treatment is what you'd expect -- back off the fat for a while and give your liver and gallbladder a chance to recover. (Should it surprise us that the French are experts on this? Mmmm... delicious FAT!)

It's possible you may already have stones, because ordinarily the gallbladder would just empty into the gut. The stones can be harmless for a long time, because they just rattle around in there until there are so many that one of them blocks the duct, at which point you go from 0 - 4000 on the pain scale, and then it's off to the OR for you.

If a milder version of paleo ends up resolving the problem for you, though, I wouldn't waste much time worrying about it.

And in closing...

"drop the soda! drop the soda! bcaw!"

Drop the soda and try switching to all natural, like suggested. Your diet is fairly similar to my paleo diet and I haven't ever experienced any problems with it. In the past, I've done a mostly paleo and still had diet dr. pepper without problems, but everyone is different. If none of those work, you really should see a doctor.