Up until i got the contraceptive injection 3 weeks ago i hadnt had a period in 11 months. they have never been regular and im 21. any idea why?

Maybe hormone iissue: No period in 11 months is called secondary amenorrhea. There are many causes such as abnormal hormone levels of thyroid, cortisol, testosterone, lh, fsh, prolactin or estrogen. Obesity, eating disorders, weight changes, and stress can also cause period problems. Being on contraceptive injection now will alter hormone levels. I suggest discussing with your doctor.

Depo provera (medroxyprogesterone): If you are talking Depo Provera (medroxyprogesterone) shots, that's the reason why you're not regular on your periods. There is a chance you would be irregular even if you were not on it just because there is a reason for you not to be regular, but Depo-Provera shots definitely knock periods down.

Anovulation: Many women have disorders of ovulation and therefore have infrequent menses. This can be due to abnormalities in various hormones such as thyroid, prolactin, androgens, and others. Now that you're using Depo-Provera provera, (medroxyprogesterone) it's okay to not have periods, but you'll probably have some unscheduled bleeding here and there.