I started taking pills after my period last 18th of May. I skipped a couple of times and not been able to take it on time as well 19th of May I had contact with my boyfriend and last 26th too but before that 3 days I have not been taking my pills anymore and yesterday I had brownish discharge that turned into red and am having menstruation again with back pain and some abdominal pain.. am I possibly be pregnant?or if not what are the other possibilities?

It could be anything - no way to know without visiting a Physician. Birth control is NOT 100% effective. Miss/skip/make up pills and you basically have no protection.

You seem uncertain whether ot not you want to have a child at this point- you are on birth control but you forget to take it.

You have had unprotected sex. Only time will tell if you are pregnant.