breastfeeding and supplementing formula?

ok so i bf and supplement formula is it possible for me to still skip periods when bf i had 4regular periods all started around 6th or 7th and i havent started yet i can say ive been mostly bf he gets 3bottles while im at school im am not on birthcontrol just using the pull and pray method so anyone? know of its possable to skip period if im not only bf i am gonna take a hpt but yea thank you?