Possible yeast infection or STD?

The opening around my vagina itches pretty badly and is kind of sore. Also it almost feels like when you have a tampon in but you didn't get it in quite far enough and you can feel it on the outside and are using your muscles to try and push it back inside. When I feel around down there, the opening feels tight and almost swollen. Started noticing it a little bit about 5 days ago and it hasn't gotten better. I had my period last week and unprotected sex the week before. I have a doctors appt lined up for end of this week.