What Could Be The Cause Of Vaginal Irritation?

I'm a 18 year old female who is a virgin but otherwise sexually active. I had been experience a little bit of irritation down there last week but it had gone away. Today, I started my period and put in a tampon as normal and everything felt fine. We were on a road trip so I wore it for about 5 hours and everything felt good. Then I wore it for about one more hour and was walking around and it started to hurt, like irritation like. I took the tampon out and instantly felt no pain, but then tried to put in a new one and it hurt again. What should I do?