What Causes Painful Blister On The Labia?

Here's an awkward question. I just found what I think is a vaginal blister. I think it's on the labia... the inner one. It's not white or red or anything. It's just the color of the rest of the skin; pinkish. It's swollen and is painful to the touch and when I walk due to friction. I'm not sure how long it has been around. At first I thought I was just raw from friction during sex a week or two ago, but it seemed to get better. I put it out of my mind until a few days ago when I noticed pain. I just found the blister today. I've been sexually active in a homosexual relationship for almost four months. Before that, I was with a man, but that was 9 months ago. I just want to make sure this is nothing serious like an std or cancer. It looks like a blister. It feels like there's liquid in it. Should I be worried? Should I just leave it alone and watch it or should I go to a gynecologist? I'm a little freaked out and just want to make sure I'm not running around with an std, though I have no reason to believe that any of my partners had a disease.