What Causes Loss Of Appetite And Knee Pain?

Hello, my name is Sabrina and i'm 21 years old. I have a few symptoms that i have ignored, but now hoping to get the answer too. One of the symptoms is that, i really don't eat three times a day like i should and when i want to eat something, i decide i don't want it and so i don't eat, until at supper or at lunch. I hardly ever eat breakfast just every one in awhile. My other symptom is that i wonder if i have knee pain/ weak legs. I can only sit/stand for a short peroid of time, before i have to get up and excerise to make the pain from my knees go away.