I have a spot under my left leg half way from my knee to my waist that keeps filling with blood and then pops. i never feel pain or discomfort. what could it be?

As above: It seems it is bothersome , otherwise u would not have asked this question. To be on the safe side , see your PCP for evaluation.

?Varicose veins: You may have varicose veins. See a vascular surgeon.

Perplexing: It could be a varicose vein. It could also be a venous malformation, a venous lake, or some some kind of lymphocele or lymphovenous malformation, as just a few possibilites. I would recommend seeing a vein specialist.