Girlfriends period is 5 days late could she be pregnant or is it to early to tell We last had sex on 1/13/2019?
What was the test result when she peed on it? It should be reliable, since tomorrow is three weeks after sex. (Testing a week after sex is ridiculous, there's no hCG even if she conceived that exact day. Two is possible, but still may be too early.)
Doesn't matter when she LAST had sex. Matters if she had it when she was fertile. If she is late, she can test.
Tell her to take a test.
Usually it is advised to wait until two weeks after having sex before taking a pregnancy test, and you should read the instruction leaflet which comes with the test. But two weeks has now long passed since 13th Jan.
Use a pregnancy test. Since it's been over a week since you've had sex, a pregnancy test will work to tell if she's pregnant. If she doesn't get her period within a week of when she should've, then there might be a chance that she's pregnant.