My chichauhaua just started walking on three legs and she won't ley me touch it.When u try to touch?

it could be sense she is a older dog that the years she has behind her are catching up cause she is getting old there are alilments they can get from arthritis to hind legs being involved medical problems can cause it to it probably hurts mabe it got injured nine times out of 10 its a older dog problem sometimes a aspirn or half a aspirn can help i would take it to the vet and get it evaluated to see if its a injury or getting old or medical issue of some sort