HELP!

Recently I have seen yellow discharge on my panty liners & it really burns when I pee. This happened the very next day I had unprotected sex with my bf. I asked him way before we started having sex if he has anything. He said no. Could this be a sign of yeast infection, STD, or something I don't know about ???? It's been just a few days & I'm making a doctors appointment to get it checked out because I can't take it anymore  It hurts!