Had protected sex with my gf, with all precautions 3 weeks ago. she got her period yesterday, but it stopped today. could she be pregnant?

See your OB-GYN: A urine pregnancy test will answer your question, since the bleeding you are describing could also be early spotting and not a shortened menstrual period. There are also many other reasons than a shortened menstrual period for vaginal bleeding than pregnancy. For the strongest result, wait a week or two until her normal time for her period to take the urine pregnancy test to decide. See your OB !