I was in a similar boat. I would recommend switching up the fruit for some starchy carbs - potatoes, sweet potatoes, bananas, that sort of thing. That did the trick for me.

Too much lean meat and fish. Ditch the chicken, go for fatty cuts of beef and pork. Also, like Pocket Groket said, add more starchy carbs. Coconut has actually been found to help with weight loss so maybe stop drinking that for a bit and if you feel comfortable, drink full fat dairy instead. Basically, if you're still losing weight eating that much, the most simple solution would probably be... eat even MORE.
And building muscle depends on your body. If you do heavy lifting religiously, won't take too long to see a difference as long as you're getting proper pre- and post-workout nutrition and you get enough sleep.

Carbs will definitely make you gain weight. White rice or sweet potatoes will whet your appetite to eat more calories.

If you don't want to do that, there is another way which may be questionable. Have some pork rinds. I've been experimenting with them to see what the ceiling of my "metabolic advantage" is when I Paleo. When I was very low carbing, I found that I would not gain weight up to like 4000 calories. Now that I'm eating moderate carb Paleo (150g carbs), the ceiling seems to have come down to like 3200 cals or so.

A bag of Baken-ETS pork rinds is 800 calories. I started eating 2 bags everyday for about a week and felt my pants tightening around my rear in 3 days. Sure enough, I gained 5 lbs.

http://www.fritolay.com/our-snacks/baken-ets-traditional.html

Is this Paleo? Sort of. Pork skins and salt. No artificial ingredients. But it seems not all fat is SaFA, which worries me a little that it may not be just pork skin toasted in bacon grease. It's hyperpalatable and addictive -- it's all salt, fried grease, and sodium. Also, too much protein. Two bags will set you back 1,600 calories and 140g of protein.

May not be Paleo but a surefire way to gain weight by eating fat/protein.

Male or female? I'm a guy with your same height and my weight has naturally gravitated down to the low 120's... it may sound too light, but I eat well. I figure I'm just genetically predisposed to be thin.

Hello from another naturally thin person from a small family. :) My little sister has your exact stats, she is very slim indeed...

I wouldn't get too worried about 3-5 lbs. You've probably decreased your BF% and put on a bit of muscle is all - though this might have had quite an effect on how you look in the mirror, especially if you tend to have lower body fat already or gain/lose mostly in certain areas (chest/shoulders are my issue).

HOWEVER unintended weight loss and hormonal disruption is a very real risk for thin people eating low carb/paleo. EAT MORE CARBS, GIRL (ETA: you are a girl, right?) There is absolutely no reason for thin/lean people to limit starch. I feel my best with plenty of carbs every day - 150-250g depending on activity level. I avoid gluten like the plague and also most grains, but I eat lots of white potatoes and white rice, also tubers and root vegetables.

I also eat high calorie and very high in fat, but I want to gain another 10 lbs. I cover all my starches and veg in fats.

If you are open to adding more dairy, cheese is a great way to add calories.

Go for full-fat raw cheese from hormone-free cows.

I agree with the general thoughts here, you need to eat more.

You're eating pretty low carb at the moment, I would definitely kick in more carbs, not only for the extra calories but they will probably make you a bit hungrier and you'll feel like eating more.

Sweet potatoes, potatoes, winter squashes, yams, white rice, etc. You could also kick some more fruit, particularly berries.

If you don't do so well with carbs for whatever reason then up the fats, more coconut milk, coconut oil, olive oil, avocados, fattier cuts of meat etc.

I find a low carb diet is fantastic for dropping weight due to it's appetite suppressing effects, whilst it's much easier to eat more on a higher carb diet. I also think this fits nicely with the idea that once a upon a time it would of been more likely that we would of eaten more carbs and calories during the plentiful warmer seasons and lower carb, lower calorie during the colder less plentiful seasons.