Last period feb 13. till now no period haven't had sex for 6 months and there's small 2 bruises one on my arm and leg. what is the cause of this?

Common things first.: Usually, changes in a young child-bearing female's menstrual cycle can be attributed to a few things. Among those "few" diagnoses, the most common are usually (1) pregnancy and (2) thyroid abnormalities. Of course, seeing your Ob/Gyn about this would give you the correct answer. Until your appointment, check an over-the-counter pregnancy test just to be certain you are not pregnant.