I have been on the pill for about 4 maybe

Patient: I have been on the pill for about 4 maybe 5 or 6 years and have had a consistent period every time,never missed one,until now that I have been inconstantly taking the pill and having inter course unprotected I have missed my period two months in a row and maybe even this time,I took a test and negative,what else could it be?

Doctor: