Knee Anguish

I don't know if this is arthritis and I was hoping someone could point me in the right direction if it isn't.

Over the last few months I have had pain beneath the patella of my right knee. It was subtle at first, but lately has been increasing. Now there is not a time when it does not hurt and sometimes a biting pain shoots both up and down my leg when I apply pressure or bend my knee. It is not a constant shooting, but always at least a dull ache. I am 21 and overweight. I already have arthritis in my hands and wonder if I now have developed it here. Or is this probably an injury of some strange sort (i have no memory of injuring my knee and because it developed over time this does not really make sense, but is, I suppose possible.)

thanks for your help, and I know this is not a medical diagnosis. I just need guidance in the right direction to discover what this is.