Nausea after drinking alcohol.

I used to drink alcohol just fine. No worries about that. Didn’t have to worry about what order I drank them in. Then one night I was feeling great but I had some vodka mixed with a Pepsi zero and I immediately felt nauseous. Didn’t throw up. I hate throwing up and feeling nauseous so I just said whatever I’ll just stick with beer because I like beer better anyways. This seemed to work and I even had some hard alcohol mixed or like a mikes too. Then one night I had 8 beers and got sick. Now it could have been because of the amount of m&ms I ate or how fast I ate the pizza but it made me puke. Next day I had 2 glasses of beer at the bar and I was fine. On new years I had 4-5 beers and I felt nauseous but I had a lot of chips and really chocolaty brownies. I haven’t drank much since but the one time I did, I had 7-8 beers over like 2.5-3 hours. I went back and I felt weird not nauseous but I think I was just worried about it but I made sure I deodar eat any thing that contained chocolate before hand and I was ok. Is it the chocolate or is it like an allergy or something? I would really like to get back to drinking normal.