Hello,
Heavy periods can be due to hypothyroidism, pelvic inflammatory disease,endometrial proliferation or hyperplasia like ovarian tumor and polycystic ovarian disease, polyps(cervical or endometrial.They are small, fragile,non cancerous growths in the cervix or uterus) and neoplasia,endometriosis,fibroids(benign (non-cancerous) tumors that often occur in the uterus of women during their thirties or forties),uterine cysts and uterine sarcomas.
In my opinion you need to get your pelvic examination done from your gynecologist along with investigations like complete blood count,thyroid function tests,PAP smear and ultrasound uterus.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
You cannot get pregnant the day before a period starts. The bleeding you describe just sounds like the start of a period. Also by having sex a week ago - she's not going to be able to test or anything yet anyway. Most likely she just had an off cycle for some reason, it happens. If in a few weeks she's concerned she is pregnant then she can take a pregnancy test to ease her mind.