What Causes Abnormal Periods And What Is The Medication For The Same?

I'm experiencing almost non-stop menstruation Sometimes too much bleeding and sometimes just a drop. Unlike before my menstruation is normal, 3 days only but now no.It started when I came here in Qatar 2007, I went to the doctor that time and they gave me medicine. Now the same problem again, last year my menstruation become abnormal. What and why is his happening to me.. ? What to do and what medicine should I take? please help.