What Causes Vaginal Tightness During Sex?

I had sex with my boyfriend and he said it felt tighter i been on birth control for almost for years now i get it every 3 months (depo shot). It usually stops my period but the last one i got in December i bleed all through January i stopped yesterday but it started back a couple hours later. Today it stopped. But my boyfriend thinks im pregnant because he said its tighter. He has two kids and said that is how his baby s mother felt but i got a pregnancy test at the doctor a couple weeks ago and it was negative so im going to take a home test today and see but i really don t know im only 18 and am confused on if i am or not. Can someone help please