What Causes Delayed Periods?

i had sex with my girlfriend on 15 sept, after thar she got period 6 days late, but she got the period . she got her period on october , and got november now she delay 2 month her period. i am scared pls help. is she pregnant?

if she is pregnant can give the correct answer urine test if she is 4 months pregnant. or can give the correct answer blood test if she is 4 month prego. pls help me.