This sounds like a job for CarieFairy.... She will know the answer. I think if it were me, and I were worried about something like this, I'd see/call my gyno nurse. I know she'd be able to see it and get it out! Good luck!

Condoms can slip off sometimes, but usually only if you buy the wrong size and/or are putting them on wrong. You should talk to him about it and see if he is willing to purchase a smaller size and read the instructions to put it on correctly.

If you are worried about another condom being inside of you, insert your finger into your vagina after washing your hands and feel around. It can't go very far up, because it will run into your cervix and can't pass past there.

If the condom slipped off and stayed inside you, I would go get Plan B ASAP (the sooner its taken the more effective it is) to prevent an unwanted pregnancy.

Why wouldn't he tell you it had fallen off and gotten inside you? As was said previously, get Plan B.