QuestionI usually have a painful sore on my vagina each month before my period. Could that be a sign of a STD?
It is a sign of an infection, but whether that disease was sexually transmitted or not depends on whether you have been having sex and whether the people you had sex with were carriers of that disease. But you can get sores from diseases that are not sexually transmitted.
Your immune system is not as strong just before your period because of your changing hormones. So an on-going infection is more likely to flare up at that time of the month.
Whether it is an STD or not, you need to get this checked out by a doctor, such as a gynecologist. You shouldn't ignore a disease because often times the symptoms you notice are some of the least damaging things a disease is causing to your body.