Well, K., I just wrote you about the "mean mommy" thing. You have a lot to deal with right now! It is so hard being postpartum, taking care of a new baby, continuing to take care of a daughter who needs special attention, and dealing with all the usual wife/husband issues.
I don't know what you should think or do, necessarily. I think Crystal was wise in saying be sure you know what you will do if you talk to your husband and it doesn't go well. Based on your other issue, it does sound at the least as if your husband is not very supportive of you right now. It would bother me if my husband was having this much contact with another woman, for any reason, and especially if he was also undermining my discipline of our daughter.
I don't think men realize how much emotional and physical support women need after they've had a baby. But just because your hormones are out of whack doesn't mean you're reading this wrong! I wouldn't blame anything on hormones to him...don't give him an easy "out." I think I would talk to him more in general terms...we need to get on the same page with what we are doing as a family...try to gauge his level of committment and openness to that before you proceed with concerns about the woman. But when and if you can, I would really work hard to find a way to end that relationship! If you can get him to a place of saying he really would do whatever it takes to give his family what they need, then perhaps you could tell him very matter-of-factly what you've observed. Don't say more than that, especially if he asks if you think he's having an affair. Just point out the facts, and tell him you would really like for him to stop having contact with that woman.
I will say this sounds a lot like an emotional affair, and if you Google that term, you will see how dangerous that can be! So don't blame your suspicions on hormones, and don't throw this all up in his face, but do listen to your instincts and keep watching...and read up on emotional affairs.
Bless you, K.. You are in a hard spot, but "this, too, shall pass." If you believe in God, turn to Him with all of this. He knows and He cares, and He will make a way for you and your family.
S.