Foot numbness

On my last two long runs (which is only 4-5 miles for me) I've gotten numbness on the bottom of my left foot. I wear whitin trail runners and have been running mostly on fire road type trails. The numbness occured on both runs around the 3ish mile mark. It wasn't painful at all but it did last until the rest of my runs. I thought the first time it was from the cold but today it was fairly warm and sunny. Any advice on how to prevent this? Any way to know if this is a larger problem or running form thing or just a temporary thing while I build strength and endurance in minimalist running shoes

(My current weekly load is a out 12miles with 500-100 feet total elevation gain. It's been gradually increasing since I started getting back into running about a two months ago)