(((candy))) You are not evil. Having times of stress and saying things that might be misunderstood or maybe even feeling non compassionate does not make you a non-compassionate person. We all have times like those, but our main core of who we are is still good.
I am sorry that you (and others) feel guilty at times. A psychologist I know would say something like, what part of your also being sick or depressed will help those other people feel better? Meaning, it's ok to feel better than another person. I haven't met anyone who really wanted others to be feeling poorly just because they do.
When you are feeling well, that's a good thing! To not be able to post to others who aren't feeling as well, I think is a common thing, too. Maybe it's a type of self-protection? Maybe it's that you can identify with what they are feeling so well but you are needing a break from experiencing it, too? There's no shame in that.
I know for myself there are times where I am doing ok, but not really well with my chronic pain, and find that I can't seem to say anything quite right because of the place I'm in. I try to find something supportive to say without saying anything that comes out negative and the words just don't come.
Well, I've rambled, but the bottom line is this: you're not evil nor non-compassionate.