Dog Suddenly Walking on Three, now Two legs...what has happened?

My dog is a 5 pound chihuahua rat terrier.
She's always been skinny/bony for her breed. She literally has no body fat on her lower leg tendons or ribcage.

A few days ago she started avoiding putting weight on her rear left leg/paw...then it went away. Now she's avoiding weight on her front left paw/leg...doing the three-leg hop thing.

Carried her outside to do her business...she was trotting at full speed on two legs! The two right legs.

I do see a laceration on the middle paw pad of the left front foot...I put some neosporin on that and wrapped it in gauze for a few hours.

Taking her to the vet for labs and xrays but...she's three years old, no litters. Normal heat cycles.

Any advice? I'm stumped. Would a warm bath in epsom salts help?