Period keeps coming early?

For the past 3 months my periods have constantly been coming early and I'll start getting cramps and things a week before I'm supposed to come on, I wasn't supposed to come on until Monday but I came on today and the month before that my period was a whole week early and when I get my periods I'm in excruciating pain which I never used to get and I've also gone from lasting 4 days to anything for 5-7.
​I have been having sex the last 3 months very often, without protection and yes he did ejaculate inside me each time. I last had sex on 21st June. I don't believe I could possibly be pregnant although I do feel like it's not possible for me to conceive as many friends have told me that these are probably not periods and just miscarriages, but I would rather have a few more opinions just to ease my mind or something.