Hello. I have a six-year-old GSD. About a month ago, he started limping on his left leg when he first tried to walk upon standing. He has significantly decreased energy in the last several days and now has not eaten since lunch time yesterday. We are giving him glucosamine chews presumably for arthritis, but I don't understand why he has stopped eating. He always had a healthy appetite before and was even a little overweight until we put him on a weight management food in January. He just wants to sleep all the time. I hate to see my baby like this...what could it be?