Girlfriend thinks I don't want to have sex with her because she had a baby....

My girlfriend had our son 3 months ago. She's been ready to have sex for a month and I haven't wanted to.... She started saying it's because I hate her postpartum body but that's not it at all... I have been going through a some depression and started putting my cigarettes out on my thighs.... I haven't even brought this up to my therapist yet. I got stressed out because I had to relive a horrible event and this is how I've been coping. She doesn't know shit about my past and I plan to keep it that way. She just thinks I see a therapist for anxiety. If she sees this she will have questions I'm not ready to answer.