What Causes Tightness In Vagina During Intercourse?

Hi this is kind of embarrassing I have been experiencing discomfort during sex as of yesterday. I have had the same by for 3 years I have never has this problem. I haven t had this problem since the first time I had sex. I seem to have shrunk or gotten tighter somehow and I pushed through it because it ended up ok but when we tried today I was so uncomfortable I had to stop. Have you heard of this before any suggestions