I can see why that is concerning. It is very unlikely she would get pregnant from fingering unless there was sperm on your fingers. It’s normal for periods to sometimes be a little ‘off’–a normal range is between 21-35 days so there is some spread. If you are at all worried about her being pregnant, which it sounds like you are, she should do a pregnancy test. Just so you know .pregnancy tests are very accurate for sex that happened more then ten days ago, but not at all accurate for sex before that—the hormone that the pregnancy tests monitor don’t show up in the body in quantities we can measure well until ten days.
There are also all sorts of other things that can cause stomach pain and anal sex can cause injury. If she is still having discomfort, she should consider seeing her medical provider.