No longer interested

So, I've like had no desire to have sex since I've gotten pregnant. And 4 months after giving birth, I still have a hard time wanting to. I feel bad cause let my boyfriend down all the time. I just feel like my head is always somewhere else and he doesn't always understand why I just don't feel like it. He tells me how I "used to like it" or I "used to be fun." and starts to get doubts if I still love him. And I do!

Is it normal to just not be interested in sex after giving birth? What should I do?? I feel bad!