Days Where Nothing Just Goes Down Nicely

I'm curious if anyone has the same issue as I have. There are some days that I just can't seem to eat anything that settles. Doesn't matter what it is. Could be a piece of chicken that is prepared the same way and it goes down fine, but if I were to make the same thing a couple days later, I can only take a few bites and it just feels like I ate a rock! And on those days it just seems like anything I eat doesn't settle right, no matter what. I usually have absolutely no appetite on these days but know I have to eat something to get my protein in. Should I have a protein shake on those days? I usually stick to a Greek Yogurt for breakfast and lunch those days. Anyone have any other tips or things that they do??