[Eternal]

It usually takes upwards of four weeks before pregnancy symptoms even begin to show up. The nausea and anything else she was feeling only one day after was completely unrelated. As others have said, the chances are extremely low. Her period could be missing due to a variety of reasons, one of them being that she is a teenager. When I was 16, my periods were very irregular, and I could go months without it at times. If you need the peace of mind, you can buy a pregnancy test, and no, you don't have to be a certain age to buy one. Pregnancy tests are very rarely wrong, but just be sure you read the directions and use it properly.

A lot of teenagers have pregnancy scares when they start being intimate with someone. I think this is a good reminder to be very safe when interacting with your girlfriend. Use condoms when necessary, and she should consider talking to her mother about birth control.