What Causes Yellow Vaginal Discharge After Depo Shots?

yes, i am (currently) on my 12th week on the depo shot. i miscounted, and wrote doesn that i am due for it next week. lately, ive been having a yellowish (brownish occasionally) discharge. ive had 2 pregnancy tests in the past two weeks, as i had lower back injections of corisone. me and my boyfriend only see eachother on weekends, and i was wondering if its safe to have sex? my las shot was 12/29/10