The bruising and history of pain in the leg are certainly consistent with trauma. If she is walking around ok, likely nothing is broken or dislocated so a muscle pull/strain or joint strain is a possibility.
With the bruising though, I would want to rule out any possibility of a bleeding disorder. The bruising you are describing could be from a decreased ability to clot. If she had bled into a muscle or joint, that would certainly cause her leg to hurt. If your veterinarian did bloodwork, he should have a platelet count - double check that this is normal. When testing ability to clot the blood, you will also need to check a coagulation profile - these tests are the prothrombin time and partial thromboplastin time (better known as PT and PTT).
Is there any possibility that she could have gotten into rat poison, even on someone else's property when you were not looking. Have you seen any ticks on her?
Check her gums, the inside of her ears, and her belly. Do you see any other bruises or small pinpoint red spots. Those are a sign of a bleeding disorder.
If due to trauma and the signs are mild, she should be fine. I don't want to alarm you unnecessarily, but in the interest of being thorough - if she has a bleeding disorder, you need to address that right away.