Since no one has answered you yet, I thought I would give it a try.

On one hand the Bikram would for sure help you lose some of that water weight.

But on the other hand, its pretty intense (what with the heat and demanding yoga), so it might stress out your body. Which is probably already under a ton of stress from the food allergies/autoimmune issues.

Then again, I hate going to Bikram. Being roasted while I workout intensely in the dark while a instructor militantly monitors me = not my idea of a good time (or a paleo time).

I feel bikrams may be a bit too intense for your body if you've gained 15 lbs water weight over a weekend. Why? Because the classes are 90 mins in a very hot room, and staying hydrated is key. Does water weight mean you are adequately hydrated? I cant say for sure. Instead, I'd recommend that you seek out yoga in a gently heated room where the temperature is kept lower than at bikrams. Speak with the studio directly and ask their opinion as their instructors should have received training specific to your condition.

Yoga creates heat in the body, so any vinyasa class - or hatha for something gentler- will do you good.

Always remember that your yoga practice is about you, so if you get yourself into a situation where you feel the heat or flow is too much, simply take a resting pose. No one is judging you but you :)

I have Hashimoto's and doing yoga helps me... but NOT Bikram Hot Yoga. Since my body temperature is a bit wacky due to being Hypo I just can't workout in such hot temps - feels like I'll pass out.