Although it might be caused by being out of shape, it might also be caused by crouching too much in your riding stance. Standing up straight is bad, mkay? But crouching too low is also bad, mkay?
If you have someone with you on the slopes, have them look at you as you ride and then have them tell you how you compare to some random person. That'll at least give you something to work from.
I hate the parts between winter