How come i jhust found out im 5 1/2 pregnant?

Ijust found out two days aqo about my pregnancy .. buh i dnt kno why im this far alonq ?ihad unprotected in march buht got my period in april may skipped june qot july 11 and now didnt qet period this month aug ... took pg test jus to take had a big fat positive and alread had a qyno visit the next day and dr confirmed .. has this happened to anyone else ? Im 20