She is young, but too old for any type of growing pains (I think).
Might be soft tissue.
Could be more of like a ligament strain.
Did you check her pads for sensitivity? Some dogs get raw pads from hot sand.
Probably take her in. I thought Sam had a ligament issue last summer when he went three-legged lame. Turned out to be a cut in the pad that was really hard to see and so clean that it didn't even bleed. Even my vet was stumped until the dog was relaxing as we were chatting before I left and he started licking his paw. Vet got the headlamp on, got on his belly and started really spreading the toes and getting in there hard. And, there it was!