I can relate to that dark heart, and I can relate to that feeling off meds too. Usually for me I think it's a low level depression, well unless I'm full on depressed. Even when I'm mildly down I have a problem feeling excited about exciting things.

Maybe at times like that a good thing to do would be to send a thank you note or card. Just to express your gratitude, that way the other person knows how you feel and you won't feel as worried hopefully or guilty about your feelings ( if you do, I know I worry excessively about those things)

And if you have a lot going on right now that really could be it, just stress maybe. When I'm stressed I think my mind sort of filters my emotions a bit so that I can get done whatever needs to be done. Just getting through the moments.