M.O.
I think this must just be a "5 yr old thing". Our daughter is the same way. I think it's just her way of expressing frustration. I know how disheartening it is to listen to!
I instead ask her "what did you learn today" or "what was the best part of your day/class"? I think by posing the questions in a positive light, I encourage her to give "happy" answers.
When she is negative, I simply say, "I'm sorry you didn't have a good time." And move onto talking or doing something else.
I have found that the less energy I put into her negativity, the less she seeks attention for it.
Again, I think it's just a phase. We did the same thing, put her in a class she begged to go into and all she did was complain and say it was boring. I just remind her of that the next time she asked to enroll in a new class. I told her that if all she's going to do is complain. And that she has to FIND something to be happy about and tell me when I pick her up.
This seemed help her to understand that we aren't going to spend money on "special" things for her if she's not going to appreciate them.
Best wishes - I hope this phase passes soon too!