[Kali_Namir]

First off, keep in mind that your knee hurts when you walk too much and don't have your brace on and make a note to make sure you've got your brace on before you go walking. Don't entirely avoid stairs, but if they are already starting to hurt, don't push them.

Definitely go to a doctor as soon as you can find a competent one that you trust. It is a possibility that it is simply your joints not being used to the amount of exercise and the stress involved with it. It is also possible that the scale is wrong when it comes to your bone and water, sometimes those don't always read correctly.

On the other side of that it is possible that you have a mild osteoporosis, or a simple low bone density, and that can be helped by taking doses of calcium.

Is there a gym or something near you with a cheap membership fee? If so, you can go in there and do elliptical and ski machines, both will give you about the same workout as walking/running, but are lower impact and should hurt your knee less.