Any of us who has gone through Osteo goes right to that place -- panic. Try not to. My first girl Scarlett did the scream thing and limp thing at one point, I couldn't find anything wrong so off we went to the vet. It took him 30 minutes of probing before he found a knotted muscle very deep in her left shoulder. Medication worked really well.
I think everyone is right and that getting x-rays will hopefully rule things out and give you some peace of mind.
Holding very good thoughts and prayers for Legs.