Possible causes of random limping in my dog?

She's a 11mo Pitbull mix, about 35 lb. About 2 weeks ago shehad her first limp at the dog park, after that its happened once every day or so. The weird thing is she limps for only a few minutes and after that she seems to be just fine. My vet told me not to worry about it unless it gets worse which It hasn't until about today. I got her ready for a short walk and as she was walking to the door with me she yelped, walked a few steps on 3 paws and went down. She seemed to be in a lot of pain but less than a minute later she was just fine again. I don't know what to make of it. I'm taking her to the vet tomorrow, Any ideas?

(Front right leg)