Get Your Dog Care Questions Answered by Experts
Hello and thank you for using this site to help you.
Because of the breed of dog Leo is I would be worried about a disc issue in the spine causing the problem. Smaller breed are prone to Interverteral disc disease which is where the tissue that cushions the disc suddenly turns hard and presses on the nerve causing the dog to have problems walking. In some cases a disc can rupture and render the dog paralyzed.
If the dog is dragging the back legs it is a medical emergency and you need to get to a vet asap.
I am going to direct you to a link for further reading on this so you can get a better understanding of what may be happening.
I would recommend strict cage rest, outings for potty only, no climbing steps, or jumping on furniture, until you can get a hands on exam by your Vet.
For today you can try giving a baby aspirin this AM and again 12 hours later. Give with food.
lower spine http://www.petplace.com/dogs/intervertebral-disc-disease-thoracolumbar-area-in-dogs/page1.aspx
Picture of disc