Hiya :-)
Ah, well I'm no doctor but it does sound like Amenorrhea (lack of periods) which can be caused by excessive stress and weight loss.
I have had this myself too and when my periods returned, they were exactly as you describe - heavy, painful, went on for just over a week.
(I don't think it is specifically linked to PCOS, - but I am no expert - there is an AB'er called 'Squad' who hopefully will pop on this post later - he is very knowledgeable on medical issues)
Good that you have already booked the doctors appointment - keep the appointment and go to put your mind at ease - they might be able to suggest ways of easing the pain. Hopefully your periods should start to fall back into a more natural pattern and become less heavy - as long as you don't lose more weight rapidly or become extremely stressed out for a while :-)
Keep checking back in to your post later as I'm sure more advice will follow. All the best xx