Answer pleaseee

If my last period was dec.16.2016 and my period last about 5 days and my period is irregular. Some months i get my period right before or right after the 16. But ass months go by my cycle changes ill have my period by the beggining then it changes to the middle the end of the momth but it hasnt vhange in months ive been getting my period in the middle of the month. When could i been fertile in dec ? Cause now im pregnant and i had unprotected sex with two people one did pull out and cum on my back the the other guy my long time bf always cums in me i need help no rude comments.