Should maybe but just to be on the safe said i would go get a test an find out an if it says no she ain't i would have her go to the doctor an go do some blood just to make sure an everything.
Hello Nikhil, hope the day is treating you well!
To answer your question, if you've had unprotected sex with your girlfriend, yes it is always possible that pregnancy could be the cause of her irregular periods. Has she taken a test yet? They are very cheap and can be found at any drug store( Wallgreens, CVS, etc ) and most grocery stores as well. I would have her take a test first and foremost, as their almost 98% accurate.
As for the skipping/irregular periods, if she's under a lot of stress ( from school, work, and especially if she is worried about being pregnant ) it is not uncommon to skip a period, or have irregular periods. I have had my share of scares in the past, and have skipped and have had irregular periods from the mental stress that thinking being pregnant can cause. I would tell her to keep calm and try to relax as much as possible until she can take a test and know for sure. I wish you both the best and hope everything goes well!