Is my girlfriend pregnant? She has had her period, but being constantly hungry and having mood swings. She doesn't have sore breast or nausea. She had her "period" slightly late. Though she said it was heavy. She has also been through a lot of stress late

Maybe. These can all be signs of pregnancy, but could just as easily be due to something else, like stress. Personal stress can throw off your menstrual cycle. .
You . You are right, stress can cause hormone fluctuations and changes in periods, as can weight changes (too high or too low of a weight). As she had her period now, it is unlikely she is pregnant, but if the period is very light she should still consider taking a pregnancy test to be sure, as some pregnancies can cause "spotting", or light bleeding. But if it is her normal period then it is obviously very unlikely that she is pregnant.