I dont have my period at all until after i have sex?

Hey im rlly worried bout sum thing. Im 18 turning 19 & iv been sexually active for bout a year now. & from the very first time i lost my virginity my periods have never been the same. Like i got 5 months at a time without one. But when i have sex rite after i have sex like 2 days after i start my period. Its like i have to have sex to start my period. Its so weird & i dont know what to do? can sum one help please???!!!
 
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