Should I be upset that my boyfriend's mother doesn't want me to go on a trip with them?

Difficult to say without more details. It appears that the mother is thinking that having you along could be awkward. For example, sleeping arrangements. If you guys are in a serious relationship, then it would be rude for her not to include you, and rude of your boyfriend if he doesn't say anything about it.

Have you met the mother? Do you get along? Is this a monster-in-law situation?

Not exactly, it may be more than fair for you to go, although I'm sure his mother thinks you may spend a lot of time together, and she might want some down-time with her son. She may be afraid she may not have much more mother-son time with her son, before he moves on. It may be a little unreasonable, but I wouldn't get to upset about it. He might not have stated how you feel because he doesn't want any tension, and he might have wanted some time with his family. =)

-Hope I Helped

Well if he is in your age 30-35 I wouldn't want you to come with me on a family trip but his mom is pry protective and doesn't want you to take away from her time with him.

I have been having the same problem, first of all, I wouldn't get mad at him, because I got mad at my boyfriend, because I didn't think that he was doing all he could to let them know (his parents) how I felt, and when I personally stated my opinion to them they told me that I could no longer go with them up north where they were going, then they invited my now ex- best friend and her family up north, but my boyfriend and I stayed here. I don't think your being unreasonable at all I know I wasnt being that way, never get mad at him, but also Iv learned to just accept the situation you got, and possibly sit down with them and ask them why and be calm about it. it may work out for the best, or it may get worse, but because I had a similar situation I thought I could help, I mean my boyfriend didn't go because this is not the first time this has happened, but it is his mother, and he loves her, so you have to understand he will be torn, sit him down and explain that this is how you feel and it would be nice if he let them know how you felt, how "we" feel, and you will be closer as a couple, believe me it works, I was closer to my boyfriend after and I'm sure you will be too. if nothing works and you split, then it was not worth it, move on, everyone has their special someone, and it may take blood sweat and tears but keep pushing through it ;)

I would be upset too if I were in your shoes. But I can understand why she wants them to go as a family. Maybe she doesn't get to spend a lot of time with her son and misses it. If you go with them, she knows he'll be giving you more attention. She's a woman too. You know how we are (as females) sometimes. :)