Eating more resulted in fermentation?
 

Quick history:
16 yo, 5'7, #155 (10-12% BF)
SS 2-3 months, eating everything I could, no problem
CF for 6+ months

Now I want to gain mass again, and I started doing that this Monday. I had eaten strict paleo for the last 6+ months. I started to drink milk and eat oats (the only two additions to my diet).
There weren't any problems Monday and Tuesday, but during the night from Wednesday to Thursday, I woke up feeling nauseous and having "egg-burps", as if my stommach began to ferment what I had eaten. I had to vomit multiple times.

What could this be?

An intolerance to dairy or oats, most likely. Try eliminating them one at a time, see what happens.

Thanks for replying.
I tried both milk and oats again, didn't have any reaction. It could be food poisoning; i tried eating some of the meat from the batch I had eaten just before bed, and it had a diseased taste to it. That might explain it. Thanks for helping out, though!