I was on my periods on the 10th of this month ,on the 28 i started having cramps and light bleeding what could be the reason?

There are several possibilities, not possible to judge without in-person examination. At your age, one of the most common causes is pelvic inflammatory disease, i.e. Fallopian tube infection. PID is usually caused by an STI, mostly gonorrhea or chlamydia. Other possibilities are hormonal problems, tubal pregnancy of miscarriage if you are pregnant, endometriosis. See a doctor ASAP. Good luck.