15 days since the start of last period and lasted 5 days. Now I'm in between and having heavy cramps and light bleeding. What could this be?

Probably ovulation. If you are midcycle and not on oral contraceptives, what you are describing is consistent with ovulatory pain and spotting. It usually lasts a couple of days. If the pain worsens or doesn't resolve, it would be a good idea to be seen by your physician.
A variety of causes. Are you on any type of birth control? Midcycle bleeding can commonly occur as a side-effect of any estrogen-progesterone combined birth control: pill, depo shot, patch, rings, iud. If you are sexually active, stds can sometimes cause this. Make sure you don't leave a tampon in too long. Simple stress can cause irregular periods (dub). It would be a good idea to talk to your obgyn about this.