Feel like I am going crazy... I might be

Firstly, congratulations on the birth of your child. Going through the process of giving birth, then c section and fighting an infection can all be traumatic while at the same time looking after a new baby. Be kind to yourself because you are going through a lot.
After giving birth, there is a drop in the hormone estrogen and progesterone, which can lead to depression. However, that should be all the more reason your husband gives you the reassurance you need, as opposed to using it as an excuse to avoid the topic. Avoiding the topic doesn't sound positive but comes across as though he is trying to deflect or hide something. Which can cause you to feel your emotions are being ignored or the issue minimised.

You've just brought a human being into this world. You need support in every way. If what you read from the messages between him and the female colleague caused you to feel it was inappropriate, that doesn't sound like you're going crazy. It sounds more like you feel he crossed your marital boundaries and are starting to identify differences in your values. Cheating comes in different forms, not just sexual or physical. But emotional, cyber, financial.

But try not to overthink or catastrophise. Get better first. Your body needs to heal. Speak to someone, a trusted friend, or a professional like a doctor or therapist to help you limit negative thoughts you may be having about yourself or this situation. And also help strengthen you back to your former self because, as you mentioned, you are confident and independent 💓