Anal sex pregnancy scare urgent

Patient: Okay so about 3 weeks ago I think, me and my girlfriend had anal sex unprotected and I ejaculated in her anus. I got a call from her saying she is almost 3 weeks late on her period. I’m honestly terrified that there was a way that the semen dripped out. When I pulled out I took a good look an nothing was coming out. She said she took a pregnancy test this morning and it was negative but she’s never been late before. I know my stepmom has been late by a whole month and a friend of mine, 3 months. All were nothing pregnancy related. Could it possibly be we’re just freaking out or could she really be pregnant? I’m only 17 and I feel pretty dumb that I did that.