My girlfriends period is 10 days late after we had protected sex. she has no other symptoms of being pregnant, but her period is late. what should we do?

Purchase and take a pregnancy test. Pregnancy tests will be positive as soon as the first day of your missed period. If negative, consider lifestyle changes. Hormonal fluctuations can also lead to menstrual irregularities. This can be caused by stress, increase/decreased exercise, weight gain/loss, lack of sleep, ovarian cysts, etc. Talk to her Dr if the irregularities persist.

Pregnancy test (better blood). If she is pregnant, you will get much more convincing signs of pregnancy. If she is not, then further reasons will be explored.