I'm so sorry to hear this. The fact is that many women experience a change in sexual desire after having a child. This is not just due to hormonal changes but many issues such as fatigue, less energy, change of focus from partner to child, etc. If you're concerned, you might want to see your gynecologist for a blood test.
I don't know if it's him or you, either. That's for you to find out. Ask yourself what's changed for you since childbirth. You say you don't feel "sexy." Is this coming from inside you, or has his behavior changed as well? Does he still act as though he finds you attractive? Do YOU feel attractive?
Next step is to talk with your husband regarding how can you make this work for both of you. I suggest talking—rather than arguing—as your first step. The most helpful attitude is “what can we do TOGETHER to make this work for both of us?” Remember to share only your feelings; don’t attack him or accuse him. This process involves problem-solving as a team. If you put your heads together, you might be able to work out a creative solution. For instance, there are lots of ways to be sexual and intimate without penis-vagina sex. How about asking to hold stroke him while he self-pleasures? My guess is once you two get used to it, he’ll find it fun as well. If this isn’t an option you care to pursue, are there other things the two of you can do that will satisfy his desire more often and begin to build up your sense of your own sexiness and attractiveness? Think of this as a fun way to get to know each other better, and it won’t seem so daunting. Best of luck to you. Dr. J