I lost a present from my girlfriend and bought a replacement without her finding out.
How did you lose her gift? Was it an accident?
It's embarrassing to say but I carelessly lost it. I wasn't paying attention at the time.
I feel you. It's ok that you lost your girlfriend's present. It was not intentional.
I know but she has been acting weird lately and I think she is having an affair.
An affair? What makes you think that? You need to make sure you have proof before you accuse people of something serious like that.