Vagina Tightening

When I have sex with my Husband my vagina tightens alot during sex. I'm not quite sure if I am cuming or having mini orgasms, but it happens often. (NEVER TOO LONG THOUGH) When I am enjoying sex my vagina squeezes without making it I have to breath through it. Sometimes it squeezes so hard I feel like I'm having contractions with labor but the non painful contractions like Braxton Hicks. Do anyone know what it is? I should know but I was always shy on asking anyone....I have asked my husband and he said he doesn't know but he can feel when I tighten up and usually when my vagina squeezes the most is when he ejaculates. Anyone??