BABY DADDY?????

okay so i had sex on April 13th with this boy , let's call him John 
i also had sex with another boy on April 20 , let's call him Zack
After John , a little while after i got my period and it was heavy, normal, wasn't pregannt , whatever 
Zack, i had sex with him and i haven't gotten my period since ,
and i just found out i'm pregnant and im about five weeks , possibly goin to six weeks. i need help. do you guys think it's zack or john? 
because  i think it's zack because the dates sort of add up but im scared that's there's a chance that it might be John.
i told zack it's his bc it makes sense , but im scared , and he doesn't know i had sex with John , cause i'm back together with Zack.
what do i do????? 
who do you guys think the father is???