My chihuahua is 4 years old and has the same problem. I took her to the vet. mine has what you mite call a floting nee cap (patella) .If you let it go the legiment can tear and the dog will lose the use of the leg or legs if both tear. you need to go to a good vet and get the dog a exam. to find out if he has a knee cap propblem or somting much less of a problem. small dog get it often do to heredity how often i do not know. It can be corrected . you need to get the dog soon. Harry
there could be a number of reasons for this. most common reason is what is called a 'luxating patella' - where the kneecap slides to the side. also chihuahua's are very good actors. they understand that if they 'act' hurt, they will get extra attention and they thrive on as much attention as possible. if you are REALLY worried, take your dog to the vet. they can confirm whether there is anything medically wrong.