What Causes Infections With My Irregular Periods And How To Cure It?

hi im having problems with my periods (im 24), they are very irregular , and when they do come it doesnt seem to come properly and the last one i had went on for 3 weeks (it finished on november 5th) and then i had infection with it aswell which has also happened twice before this year, have been to see gp all these times but they give me antibiotics and it clears up 3 or 4 days later, but it has started again nearly 4 weeks later(4th dec) and got antibiotics straight away but this time it has not gone away, i had cystoscopy last year and ultrasound and also a kidney scan but the results showed nothing abnormal. but there must be something wrong if it keeps happening and i cant just keep taking antibiotics all the time there must be a cure to this surely?