What Causes Irregularities In Menstruation?

Hi I am Sarah. I am 19 years old female, 5'4" about 140 lbs. I have not had my menstual cycle in four months and I have the IUD in my arm, the implannon, and it expires in March 2012. I work cleaning pools everyday and drink about 3 redbulls and 2 monsters or vice versa to keep me going. I am ADHD and take one adderall, 30 mg, a day. I feel wore out and tired every day and I have a low iron problem. I bruise easy so I take iron supplements here and there to help the black under my eyes. I just would like to know why have I not had my period? My last one was March 4th and in April I had three. My cycle has never been right but this is the longest I have been without one and I am gaining weight but nit sure if it is muscle weight or blood backing up weight.