Help please

Ok so I need help. I've been worrying for a long time. My boyfriend and I have are sexual active. I'm on the pill, and we don't use condoms. When we first stared having sex, i had a normal period. But ever since we had sex more often my period isn't like it use to be. Much shorter and not as heavy. Sometimes I'm worried I'm pregnant but i end up getting my period but it's still worrying me. Please help.