Is it hard to get pregnant when you have a irregular period..

It is harder to plan for it, yes

My girl friend is 16 years old. She has a irregular menstruation. She feels sleepy, carving for food that she doesn't wants.
She said the her last mensuration is Aug. 28.
We have sex on Sept. 22. But she doesn't have her menstruation on the month of Sept. now she feels like that? Is that the symptoms of pregnancy?

And if she doesn't have her menstruation she can still be fertile ?