What can cause tired, weak, jelly-like legs?

[NyaCat]

For the last three days, my legs have been feeling weak and stiff. I can still walk fine, but it feels like it takes more effort, especially when going upstairs. I started noticing four days ago and chalked it up to being my anxiety flaring up since it has caused this once or twice before, but I didn't think anxiety could cause it to last more than a day. I haven't noticed anymore symptoms apart from being fatigued, but I think that is mostly from going to bed later than usual. I hardly ever exercise and have a mostly sedentary lifestyle, but it has never bothered me before so I'm not sure if it could be that or something worse since so many things have this symptom. Please help!

[2017mary]

Maybe you're anemic.  Can you go to the doctor for blood work?

[NyaCat]

I'm hoping to get an appointment sometime in the next few weeks if it doesn't go away soon, all of their appointments were full this week.

[NyaCat]

It also feels like I'm short of breath, but I assumed that was anxiety.

[NyaCat]

I also forgot to add that I'm 16, female, and haven't had any major health problems before.