I think you are being silly and foolish and may end up losing your fiance over your suspicions.
How can you say that? You don't live with the guy, and know nothing about him. You don't know if he is acting strangely.
If she has suspicions she could be right. She could be wrong who knows.
I don't think its foolish though. Women that I know that suspected something was going on, and did some investigating and checking found out exactly that "CHEATING" while all along hearing that exact thing "you are being paranoid"