I am dealing with a similar thing with my DD who turned 2 last month. Little things can trigger her and, with her it can be as if she doesn't even know what she wants. She is upset if I start to do X, then gets more upset if I stop doing X. It is really exhausting.
I am not sure that I have anything to suggest with the question about crying and boundaries. I do know that one night I just held my daughter very tightly while she just screamed and cried for 45 minutes. Instead of trying to fix things, which is what I normally am doing, I just tried to hold space for her and let her know she could cry as long as she needed. She really seemed to need that. She slept so well that night, but it didn't make things a whole lot different the next day.
I got "Playful Parenting" from the library and have gotten some great ideas there too. Like when she is just beside herself and we really need to move on, I try to do the most goofy thing I can think of... like putting her lamby on my head and calling it my new hat, or just whatever seems ridiculous in the moment and makes her smile. It is like a pattern interrupt and can help to shift her mood from tears to laughter (which I believe is just as cathartic for her but less stressful on mom).
It is probably just a phase, or molars starting to come up, or any number of other things that these little ones go through. Still it is really so hard and pushes our limits. Hang in there! You are not alone!