I rarely sit- seriously- but I do think i feel worse sometimes sitting

Sitting for long periods definitely makes me worse but like you I have to sit all day at work.  I use a cushion when travelling and at work that is actually for women who have just given birth and people with coccodynia (which I suffer from as a result of the IC), this does help.  I got mine in the UK though but I think the ICN sell sometihing similar in the US so perhaps try them.
Standing for long periods is just as bad for me too, keeping on the move is by far the best way but difficult to do when you have to work and have an office or sedentary job.