I just had a cycle for 16 days now i back bleeding what would be the reason for me bleeding again?

See a doctor: To be able to say for sure you need a full history, exam, and appropriate testing. There can be many causes including thyroid problems, infections, cervical or endometrial cancer and anovulation due to stress, weight or exercise changes or dieting. Only by seeing a doctor will you know for sure. Hope this helps some and you are better soon.