Over time peoples body's tend to change. What once did not affect us before, may all of a sudden affect us at any moment. Sometimes it is unexplainable. But have her bring it up at her gynecologist and see what they say.
Can't say, would have to see a Dr.
I agree with you it's just why does it only happen with me and it didn't used to happen when we were dating.
It does not sound like an std. It could be that she is extra sensitive. If you have stubble, and with the rubbing and friction, maybe she is getting rashes from that.