Just to stick my oar in...
.. you confront her. She say's 'no - all innocent. Secretly stored number is perfecly legitimate...'
... Then what?
She feels you dont trust her. Would/could you accept her response that it was all innocent?
Do you trust her?
What answer do you
really want to hear?
a: "
Yes, I was cheating" - you can work it out from there, one way or the other.
b: "
No I wasn't cheating" - there you have trust issues. She's either lying or you just can't trust her. Bit of a tough walk back from that one.
Sit on it until you have something more concrete to go on, either way. Perhaps slip some story of a mate catching his g/f cheating on him into the conversation. See how she reacts. If this bloke is a workmate, see how she acts after the 'office party'.
As far as the amature PI malarky goes; I'd knock that on the head. Taking the odd sneekey peek at her phone is one thing, electronic evesdropping and hiring a PI is a whole other world of icky poo that you'd do well to save for more drastic times. Like murders, alien abductions etc..
For now, look at things objectively. Ask yourself 'Am I paranoid? Am I only just noticing things that have always been there (new hair/clothes/odd smells).. When you're sure, either way, do as others have said and talk about it. Be calm, open minded and prepared to hear a conflicting version of events.
Facts you can work with. Asumption is the mother of all ****ups.
Best of luck, pal.