What Causes Irregularities In Menstruation?

Hi,
I had regular periods till August,2010. I dint get periods in September,2010. So i thought i would be pregnant and went to doctor. She confirmed that I am not pregnant. She gave me Duphaston tablet but then i got periods within three days but she gave tablets for 7days. Then she took all the test and scan, but all the results are normal. She gave me Saphine and Duphaston for 3months. But unfortunately i got 3months periods regularly. Jan19th was my last periods. Then again Feb 1st to 7th i had periods. Doctor gave me some tablets. Then again Feb16th i got my periods and doctor gave me Novelon tablets for 21days. Then in March 15th i got my periods. Then again March 28th i got my periods. I went to doctor on March 30th. She has given me tablets Sysron-N, Gynae-cvp, Siocare and Ferox tablets. She said I may conceive by taking the tablets or may get periods if the tablets are over. She has given tablets for 15days. So please let me know whether i can get conceived by taking these tablets. What could be the reason for continuous periods?