Conveco - what you feel/ felt isn't actually that unusual, many women feel this way after babies, not all of them have affairs but that could well be because the opportunity doesn't arise.
Having a baby is a weird time for all IME. You did what you did, it can't be changed but you can move forwards.
What was your relationship like before baby? If it was good, it can be good again and you may have a touch of PND. The sex drive often disappears - I lost mine down the back of the sofa years ago and it only reappears when i drink alot and on other sporadic occasions through the year.
It is a good idea to try to spend one night a week/ fortnight with your dh, cook a lovely meal - get rid of baby for a few hours (if poss), if not try to arrange a meal round a quiet/ nap time, get some really nice wine/ champagne, pamper yourself and seduce him. You will enjoy it when you get going . I think the body sometimes needs reminding..... he also needs reminding that you are a woman and need attention.
As for telling him, well I agree with W&B but also the other posters too, it's a difficult one to be sure. I think that ball is in your court and only you know dh well enough to know how he would react, don't make any hasty decisions though. Once something like that is out in the open, there's no going back.
If there is no way he will ever find out and you are serious about saving the relationship and do genuinely love him then it may not achieve anything by telling him. I would ask yourself to put yourself in his place though and consider how you would feel.
Either way, whatever you decide to do, you are not alone and you're certainly not the first to make a daft postnatal mistake honey.