cat having problems climbing stairs

Hi, our 8 year old cat has been having difficulty climbing stairs the
last couple months. She can go downstairs okay but she acts as though
it is an ordeal to climb up. She always uses one front leg and sort of
hoists herself up. The vet checked her over and he found no signs of
pain in her legs or shoulders. Thinking it might be arthritis, he
prescribed Cosequin. I know it hasnt been long enough (two and a half
weeks) for the Cosequin to do much, but on some days now she seems even
worse. For the last couple days she wont even try to jump up on the bed
or sofa, although she still goes up the top of the cat tree. Her
technique, however, is to sort of climb up the side of the cat tree and
then do small jumps to get to the top. She still climbs stairs when she
wants to but seems to avoid it more--she will sit at the bottom and
look up for a while before attempting it. However, she growls if I try
to help her--which is not unusual because she never did like to be
picked up.

She had a geriatric panel (bloodwork and urinalysis) about two months
ago and everything was normal. She is asthmatic but it is controlled by
Flovent. I have never read anything about that having possible side
effects. We plan to take her back to the vet next week but does anyone
have any ideas about what this might be? I am not sure it sounds like
arthritis, and if it is I cant figure which part of her, legs or back,
is affected. The vet did suggest she could get around better if she
lost weight, which we realize, but she is only about a half pound over
her best weight and she used to weigh a pound more and didnt have
trouble climbing or jumping then. (sorry about the lack of apostrophes;
google groups wont let me put them in for some reason.)
-yngver