help?

I’m in college now, and for the past few months I’ve been unable to take shots. As soon as I have the shot in my mouth, it’s like my body is rejecting it and I feel like I’m going to throw up. Last time I tried I involuntarily spit it out. Before that, I could drink vodka like it was water, and I didn’t even need a chaser or anything. I’m just wondering why this has started happening and if there’s anything I can do to fix it?