What happens when your menstrual cycle is way early? (my last full cycle was 15 days ago. no warning signs on this onset)

Bleeding? Spotting?: The average menstrual cycle is 28 days, but the normal range is 21-35 days. A cycle of only 15 days is short. If bleeding on day 15 was more spotting, & you are not using contraception, then implantation bleeding associated with conception would need to be considered. Other causes include significant weight loss or gain, stress, strenuous exercise, eating disorder, medications, & peri-menopause!

Need more info: The way female hormones cycle is very tricky and subject to many things. I would suggest that you see your OB/GYN for an exam. This early menstrual period could be normal but may be a harbinger of something serious going on.