What Causes Mid Period Bleeding After Missing Contraceptive Injection?

Hello Doctor, i am 27yr old and i was supposed to go for my contraceptive injection last month but i did not,i had periods a day after my contaceprive date and its been 2weeks now and i got my periods again,is everything ok,could my cycle be changing...i am sexually active and i have'nt been using protection because my fiance and i are trying to have a baby