Should I Have Said Something

It's not even really her this time, it's my husband. My stepson had a field trip today and my husband went, when they returned I asked my husband how did he go and he started explaining about how the other children were unruly and so I asked him if he was the only adult in his group. He said no but he didn't elaborate on it. I found out later through my stepson that his mother went on the field trip as well. I wouldn't have been so upset had my husband told me she was going in the first place, but the fact that he lied by omission has me really pissed. I confronted him about it and he tries to turn the tables on me saying that I make it hard for him to tell me certain things dealing with her. I feel like now if he is going to keep small things from me like this, then what else would he not tell me??? I don't know if I can deal with this much longer, yes I should have thought about this before we got married but then again I thought he would make me feel better about this situation too. I'm confused.