Large lump in my vagina

I'm 19 and I just started my period today. When I went to put a tampon in, I felt a really bad, sharp pain and realized I have a large lump in my vagina that feels a little larger than a quarter. It only hurts if I have a tampon in. It's smooth and feels squishy but more solid in the middle. I've always had heavy periods and bad cramps but no pain during sex. I'm going to call my obgyn, but any ideas of what this might be? I've never had anything like this before. Thanks!