Prednisone is seriously psychoactive.
It can cause depression, anxiety, OCD behaviors, mood swings, personality changes and mania. The most severe form of this side effect is called Steroid Psychosis. I had it a few years ago - not fun. I did go back to normal - for me anyways
Doctors aren't suppose to (tho they do) prescribe steroids to Bipolar patients - or even their family members - because of the high risk of Steroid Psychosis.
If it's medically necessary to be on a steroid, a low dose lithium might help.