OP, I agree that this is not unusual. The year of having a baby is a really tough time for a couple. It is very hard to get used to a postpartum body. You sound like a really nice person.
Having said that, it sounds like your wife has two things going on that need to be addressed:
1) Pelvic floor dysfunction or a prolapse. A lot of doctors miss this diagnosis, so if she just went to a regular OB/Gyn or a general practitioner, they might not have been able to correctly diagnose it. She should see a pelvic pain specialist. You could try the pelvic clinic at GW. I also recommend Fran the physical therapist.
http://dynamiccorewellness.com/meet-frances
2) Post-partum depression and/or anxiety. I had this and it was really tough. The paranoid idea about you having an affair points to anxiety. If she will consider the possibility, she could try therapy or a small dose of a med. If not, I recommend acupuncture. It did wonders for my anxiety and overall health.
It sounds like she (and you) feel disconnected from each other. Have you thought about playing cards together before bed? My DH and I do that sometimes. It's a nice way to spend time together without the pressure of a conversation or an official date.