My chi-weenie puppy is limping.

Question
We have a 7-8 month old Chi-weenie puppy named Charlie.
This morning he was running around and being crazy with our other dogs.  We heard him yelp, but he seemed find.  For most of the day, our normally VERY hyper pup was just wanting to sit on my lap and hid his face in my arms.  Then he started limping and now he is not putting any weight on his back right leg.
I have rubbed his leg and straightened it out etc. and he only seems to cry when I put my hand on the bottom of his foot and gently push up.
Could this be a sprain?
If so, how could I help him

Answer
Is his knee cap straight? These types of dogs are notorious for having luxating patellas, which means the knee cap get pushed inward toward the other leg then they can't straighten out their leg.
The other thing he might have done is torn his ACL, or anterior cruciate ligament.
It could just be a sprain too, and if so just rest and maybe a bit of ice on the affected area for just a few minutes.
In our experience however, little dogs playing with bigger dogs usually rupture something.
You really should take him to the vets if he isn't walking on it by Monday.