I have a muscle or something that kid of blocks the opening of my vagina. it isn't painful to have sex, but it's a tight fit. i can move the block, but it sort of feels like a cyst or something.

You are describing..: You are describing and intact hymen which is indeed the tissue that blocks the vaginal opening prior to any sexual activity. if it is painful or you have recurrent bleeding withi intercourse, you with need to see Gynecology

Epithelial inclusion: Sometimes a cyst can form in the vagina and it's called an epithelial inclusion cyst. It's not going to hurt you and it's unlikely to be cancer. Sometimes they form because of some form of abrasion/trauma/stitches. Sometimes they're there at birth as remnants of developmental glands. If it isn't going away and bothering you, see a gynecologist for removal.