What Causes Irregular Periods, Abnormal Bleeding And Back Pain?

Hi, I'm 34 yrs old female mother of 2. after I gave birth with my l2nd one last 2006, my period started to become irregular , the bleeding was so heavy with big clots. Last 2010 june-july I didn't get a period. But this year 2011, I didn't get my period since May. I bought pregnancy test but the result were negative. After 4 months, i felt milk came out of my breast, and it did happened twice. So I went and see the Doctor. She did the urine pregnancy test, negative and did a bloodwork regarding pituitary but also came normal. She gave me pills which I took but stopped because I was having chest pain with it and a really bad nausea. So after drinking it for 4 day only, my period start coming 8/19/2011 which is not heavy ( to decribe it probably you can change every 4 hours) and now the 22nd and I believed it is going away.
But today(22nd) iam having a bad lower back pain ... what is going on with me?
With all the things done, can you rule out eptopic pregnancy?