Missed period, but still have PMS?

I've always had weird, irregular periods, but things got better after my daughter was born 18 months ago - my periods have been mostly regular since then. I recently lost 30 lbs in a very short amount of time, and from what I've read it sounds like rapid weight loss can effect your menstrual cycle. My last period started on July 30th, so I'm currently 49 days out - about 20 days late. I attributed this to my weight loss, and didn't think much about having skipped my period...until this week. I have been SO cranky and emotional and PMS-y. Worse than usual, it seems. I don't usually have PMS too bad, but this month it's been awful. I'm also under a fair amount of stress, and have been for the past year or so with job/school/family issues. But this feels different. Like, I realise I'm being a b*tch, but I can't seem to stop myself. Does anyone know if it's normal to still have PMS symptoms when you don't have a period?

Just to be on the safe side, I took a pregnancy test 2 days ago and it came back negative. I'm not on any birth control, and am sexually active with my husband, but we're careful - rhythm method, withdrawal, etc. Plus I don't have any morning-sickness type symptoms that I had really bad when I was pregnant, so all of those things combined makes me pretty certain I'm not pregnant.

Just want to know what's up with the crazy emotional roller-coaster I'm on this week!