Diagnosis i guess...and when should a woman take a pregnancy test after her period?

my best friend lost her virginity about 3 weeks ago and she thinks she's pregnant i think its stress because she just started college and she's probably scared of what her parents might think. Her period also came on time but she still wants to make sure just in case, the very next day she was feeling jittery, her stomach was growling even after she just ate, and constant burping for no reason, but now she still has the burping for no reason but for like a week she's been having headaches every once in a while, could it be stress form school or gastric reflux (maybe) or pregnancy? And should she takes a pregnancy test 2 days after her period is finished?