ACLs tear; luxation of patella?

[Doc E]

It's extremely difficult to diagnose over the net.

If there is little or no improvement at this time frame, it's probably more than just a pulled muscle. If the xrays showed nothing, and the medication didn't work, then it's likely an injury to cartilage/ligament/tendon.
If it's a luxated patella, it should have shown on xray. If it's an ACL, your Vet should have done an orthopedic test called the "drawer test" and it would have shown up -- UNLESS it's only a slight tear, then the drawer test might seem alright.

If your dog is still having a lot of trouble, I'd be seeing a Vet Orthopedist -- or taking your dog to the nearest Vet School.

If it is a torn ACL (often times called CCL in the Vet world), there are two kinds of surgery. One is the older traditional ACL repair and one is called a TPLO (Tibial Plateau Leveling Osteotomy).
The TPLO is more expensive than "regular" ACL repair and worth every penny.