A., I am the kind of person who drags my feet a bit about seeing the doctor, but in this case it would probably be worth it, if nothing more than for peace of mind.
A few years back when my husband and I were trying to have our first child, my body suddenly stopped cycling. I took test after test thinking I was pregnant. I was really moody, periodically nauseated, and I was always sure I was pregnant, but I wasn't. For six months it was like I was in a holding pattern until I finally called my doctor about it. When he saw me there was nothing wrong, and he simply prescribed something that jumpstarted my system again. It was no big deal.
Anyway, I wouldn't let it go -- there's no sense in being stuck in PMS for months on end.