I see noone else had an answer for you - I had aseptic necrosis in my knee after steroids for asthma. I had no signs until a slight pain in my knee - I thought I'd pulled a tendon, but an MRI showed necrosis. Since I was only 40 at the time, they didn't want to do surgery, which I will be getting this year.
Talk to your doctor about this - it can be scary! You may also want to google avascular necrosis.
Good luck - hope you don't have it
Proud to be Krista's Mom