I am not a medical doctor, nor could a medical doctor tell you what is going on without conducting an examination. You must tell your mother about this (you can leave the masturbation part out, just tell her that your periods seem different) and let her make an appointment for you to be examined by a gynecologist. Your mother won’t be in the room and you can tell the doctor the truth. Don’t remain scared about this, but have a doctor look into what is going on. I don’t think it’s anything serious but it could be and the earlier you catch a problem, the better the outcome.