What Causes Delay In Menstruation Other Than Pregnancy?

I'm almost 5 months late on my period... I have taken multiple pregnancy tests and they're all negative. I just keep waiting to have my period, it just never seems to come. I feel like my belly is growing, within the past month there has been multiple people ask me how far along i was, but all the home pregnancy tests come back negative. I do not have insurance to go to the doctor to find out other wise.