What Causes Missed Periods For 5 Months With Only Cramping And Light Spotting?

I'm a 22 year old female. This is the 5th month I have gone without a period. I am not sexually active, and I'm taking loestrine 24 birth control pills. Typically I have mild cramping and light periods when I did have them. I have been experiencing nausea for the past 3 months on and off, recently nearly everyday. I have dull pain in my abdomen/pelvic area. 2 years ago I had a similar type of pain and I had my appendix removed, only to find a small carcinoid tumor in the tip. I've had an ultrasound and ct scan recently that did not reveal anything particular. Any suggestions as to what could be the problem? (please and thank you)