Semen burning throat

Whenever I do oral with my boyfriend and he finishes in my mouth it kind of burns. Not like unbearable, but a slightly uncomfortable warmth in the back of my throat and my tongue kind of goes numb. It’s also incredible bitter. There are no problems vaginally, like it doesn’t burn if any semen is there or on any other part of my body, just my mouth. Any ideas as to what that is?