Irregular periods.

Hey guys so my period has been super irregular this year (and technically part of last year). I went 4 months without having my period (no I want pregnant, I took 10+ pregnancy tests at various times and all were negative), I had my period twice after that then didn't have it again for another 4 months (again, I took tests, all were negative). I am sexually active, I'm not on a pill or anything but we use condoms 100% of the time. When I do have my period, its excruciatingly painful for the first day then the pain fades and my period is only about 4 days long. I don't know what could be causing it and don't want to have to talk to my step mom about it because we aren't that close. Any ideas?