Laxity in these cruciate ligaments usually presents as dog initially hopping on 3 lags, then toe touching.
It is also important to remember that many
small breed
dogs, including Chihuahuas, which I personally raise, are "babies" and do not tolerate discomfort very well. When they have a ache such as a loose knee ligament, often they just stop walking and act in a very similar manner to that which you have described for your dog.