whats causing my skipped periods

i have had no period for 3 months now, my cycles have been irregular since i miscarried almost 2 years ago but they have come every month, i have done 1 pregnancy test and it was negative, i often feel sick and lightheaded with some pains in my pelvic area on times, i feel bloated on times and so tired even if i sleep for 12 hours i am tired within a few hours, i have had scans, blood tests, blood sugar tests and blood pressure tests. Any ideas please.