Your girlfriend would not be cramping from any pregnancy a few days after sex. It is more likely a symptom of the Plan B that she took.
If you read the Plan B insert, you might be relieved (or not) to know that her period is going to be weird for up to 3 months after taking it. Plan B is hormonal and affects a woman's normal cycle. She may bleed early, or late. No one can tell you how her body will react.
You did the right thing in your case by getting Plan B. Although I would still advocate the use of condoms. There are non-latex brands of condoms if your girlfriend is allergic to latex.
If your girlfriend's period is late, and expect that it will be, have her take a pregnancy test. They offer them for FREE at Planned Parenthood clinics (along with birth control advice, birth control pills, condoms, Plan B, pregnancy counseling, etc).