5 Month old - Sudden Limp Onset

Does any one have any idea what causes a 5 month old puppy to limp all of a sudden? She was fine yesterday, and came inside for the night with no issues. When she next got up, about 9 pm, she was limping. Refusing to put her paw on the floor. Fast forward to 4:30 am this morning. She's still refusing to put her paw on the floor, although she is walking. We thought it might be a muscle strain from playing - her and our oldest puppy roughhouse quite a bit - but her leg looks completely fine and when running my hands along it I can feel no obvious bumps or tears - then again I'm not a vet. I checked her paw, thinking maybe it was that and not her leg, but nothing looks broken or swollen - I'd never know something was different if she wasn't limping. Other than the limping, she's physically fine - breathing normally, no heat coming from the leg or paw itself, and she's not crying at all, either when she walks or when I try to touch her. (Which, come to think of it, is highly unusual - she's a very anxious dog.) I'm stumped - thinking it may be a muscle tear or something that isn't immediately obvious, in which case, it's just bed rest for a few days until she heals. At what point do I need to be concerned? Is there anything that I've said that sounds like something more serious? If it is a muscle tear, are there things I can do to help speed up recovery or ensure she heals properly? Apologies for the long post. She's only my second baby (one of them) and I just want her to be okay. Picture for tax - she's the blonde.

She looks lovely! Sudden onset non-weight-bearing lameness is always a worry. Muscle tears usually cause very obvious localised pain and swelling, and while, a paw or pad injury is a common cause, normally I’d expect you to be able to feel that. In her case, I’d be quite concerned about a torn or ruptured ligament, or even a fracture. I really think a vet check ASAP is indicated here. Good luck!