T'fillin when sick

Do you lay T'fillin when sick or if you is in pain or sad? My understanding is one's thoughts should be totally positive and one shouldn't don T'fillin when one's thoughts are not totally on his prayers.
I wore mine the other day during morning prayers while having stomach bug and often my mind kept going to how bad I felt, so took them off.

There is a commandment to don Tefillin, no doubt. You are obligated. So don Tefillin, say Shema, and teke them off for the rest of the service.