6 month old puppy is suddenly refusing to go up or down steps in and outside of apartment?

I, at first, thought she was becoming scared of outside my apt building (I live on a decently busy street), but I’ve been thinking, and she is completely normal once I bribe her with treats off of the few steps outside. she needs treats to bribe her up and down the stairs in my apt building (I was carrying her down all the stairs to save her joints until the last week), but she used to race up the stairs, and down if I let her.

Could she be in pain? Or maybe it is just her new fear of the outdoors? I’ve had her for a little over a month by the way, but she has seemed very well settled in. I know stairs aren’t great for puppies’ joints, but I live on the 4th floor, and from what I read, going down seems to be the worst for them, so I was hoping it would be okay to let her go up. And I’ll go back to carrying her down.