What Causes Abnormal Menstrual Bleeding?

i am a 39 year old female,none children , eat healthy & drink at least 8 bottles of water a day , i very rarelydrik pop or juice & never drink alchol , i smoke around 6 ciggerettes a day , i am currently having a period & it has lasted 8 days , i am worried as this has never happened befor , i bleed normally each month & never had any problems , i was at th e A&E 2 nights ago with pain in my stomache,back & legs they have giving me 14 trimethroprim 200mg to be taken twice daily & said i had a water infection , will this be the cause of my bleed ? is this normal ?