So many periods in month

Ariel

So i been having a period everyother week sometimes i will get 5 day break then starts back up again or have a 3 day break and bleed.for 2 then no bleeding for a couple days and start up again. I am on birthcontrol pill for 18 months but the past 3 almost 4 months its like im always on my period. Does anyone might know what is wrong or anyone else that experiencing this. I feel like something is wrong.