Why do I feel guilty?

So I was on campus the other day, and outside of the business building I found $40. This girl walked up and took, and started thinking for a bit. She then said she would take it to the list and found in the library. I asked if she wanted me to take it there, but she looked at me in a suspicious way (presumably since I was darker skinned) and said she was heading there anyways. So I was like whatever, and left. I didn't really trust her though, so I went to the library after wards to make sure she returned the money but they said they hadn't received anything. I feel kind of bad now, kind of responsible because I encountered the money first and would have returned it to the library. But now somebody (who is probably broke since all students) won't get their money back because some girl ended up stealing it. I know it's not really my concern, but I feel responsible, and I feel as if I should have insisted to return the money my self.