Late Period

I am posting this for my girlfriend. We had unprotected sex the day after the period ended in March, and she was a little wary, so we went and bought Plan B. She had the bleeding from Plan B as said that weekend. Her next period then came 35 days after her last regular period. We had protected sex about 4 times this past month, nothing went wrong or anything with the condom. But this month she is now once again late. Her period was suppose to come the 5th 6th or 7th but it has not came. She is usually irregular with her periods as is, and she is a little underweight, and is stressing out very badly right now. Also she is just recovering from being sick. But she has not been able to sleep the past 3 nights.

I was wondering, can Plan B make your 2nd period late also, or go haywire, also does her stressing about it and being sick help? Can someone please help us out, I am really worried about her!