(So this last week has been absolutely appalling for me, pain-wise, and so I am dealing with it by transplanting it onto characters I love)
Sherlock suffers from chronic joint pain. Whether or not this is tied in with any other conditions/illnesses (nausea, migraines, hypermobility, etcetc) is entirely up to you. He's still the same Sherlock we know, but this condition is very much a part of his life, however much he tries to pretend otherwise.
Caring John going into doctor mode to look after Sherlock on a bad day.
Would be interesting to see it played out where Sherlock is shoot-the-walls-bored-out-of-his-mind, but is also unable to move because of the pain.