Suggest Treatment For Irregular Periods And Pain

I am 38 years old and was diagnosed with endometriosis two years ago after having laparoscopic surgery. Endo was found on my uterus, bladder, intestines and ovaries. Despite going back on the pill, my cycle has been irregular - anywhere from 11 - 34 days. I missed my period last month and I m currently on day 50 without any signs I will be getting my period soon. I ve become increasingly lethargic and consistently feel a dull ache around my lower abdomen that becomes momentarily painful. Enduring pain during my periods was normal; this fatigue and dull ache are not. What could be causing this and the missed period? Should I be concerned? I am not pregnant.