Hi,
It's perfectly normal to feel a bit upset when you don't get invited along, (Ive seen both sides of that one) but don't let the situation get to you, as others have said there may be many reasons for this.
At the end of the day whats important is the arrangements you share with your husband, and don,t be too offended when occasions like this occur, because your going to find other situations that either you or your husband won't want to participate in. So from my perspective, don't concentrate too much on feeling upset about it and move on to the activities your both comfortable with.