Boyfriend

Okay so my boyfriend was locked up for Almost 4yrs and we have been together 7. In his letters he told me the sweetest things and apologized for being a total jerk during the year before he went in. I absolutely can not stand weed and sorry if this offends anyone although I don't see why it should. Anyways he promised to never do it again. Well last week I found it in his shorts when I went to do laundry. I asked him about it he admitted his mom gave it to him. Yes I know his mother is not the best mother out there. But after that everything I said he just told me OMG, I don't know what u want me to say. We went two days with me not talking to him and it didn't even effect him. Now every time he goes to walk the dog or go to the store I find my self really irritated and I don't trust him. I feel like I was lied to and I can't trust him. Am I over reacting? Or am I just emotion and over blowing it due to pregnancy? Please I don't need any negative comments either. I just don't have anyone to really talk to anymore.