throwing up for no reason?

so this morning at school I was finishing up a paper before class started and I was sitting with my friend and all of a sudden I started throwing up. I had only drunken tea this morning, I didn't eat, and I didn't feel nauseous prior to my throwing up. I felt it a little in my throat but my stomach felt fine before, and it still feels fine now. I have a sinus infection but this shouldn't illicit throwing up. has anyone ever experienced something like this or maybe know why it could've happened?