Irregular periods when I have sex

So for the last two years my periods have been weird. When I'm single my periods are perfectly regular. Last five days.. Bam perfection. When I'm dating and sexual active my periods are heavy, last 7 days and I spot for the entire week before some dark brown blood and sometimes after sex I spot. I've got to the doctor and they have tested me for infections and what not and it all comes back fine.. I have had the worst luck with bc including periods that last two months straight. So I don't like to be on it but I'm once again considering it just so my period is more regular. Anyone else have problems like this? I'm