What is happening to me?!

Baby is 12 weeks. Ever since she was born, I've been very irritable and basically put off by my husband. We always had a great relationship, now, we can't even talk without arguing. We've had sex only once since baby and I even get annoyed when he tries to initiate. Baby is good, a great sleeper and basically no problems, so it can't be that I'm overwhelmed by her. I don't know why I'm behaving like this and feel this way. I'm so scared I'm ruining my marriage.