What Causes Numbness In Foot While Working Out?

I was working out today and my heart rate got up to 170 for a little bit, I began experiencing numbness in my left foot. I also have a bad vericose vein on that leg from my pregnancies (I am not pregnant now) the numbness went away after I stopped the cardio. And about an hour later I began to get a headache. Is there anything I need to be aware of ?