I'm not sure if I'm meant to be a mom

It's all I've ever dreamed about for as long as I can remember. Now that I have a child, who is 7 months, I just don't know if I should have done this. 
I'm a SAHM, I just feel like the worst mom
Ever right now. I have been yelling and crying for the last 4 days. I'm in such a bad funk and idk why. 
I really just want to smoke a cig to calm down but I quit over a year ago and promised myself I would never pick one up again. Plus I breastfeed. 
I just feel like I'm going to cross a line with my child and I really don't know what to do anymore. I have no outlet, I have no family or friends I can lean on. 
I'm afraid for my son right now. I haven't hurt him, but I feel like if he cries again I might, it makes me feel so evil. It's just not right. Something is wrong with me.