Could be an iron deficiency or B-12
I have the same thing.You need vitamin B12. Buy the kind that dissolves under your tongue. I haven't found any except for www.swansonvitamins.com/Vitamin B-12 Lozenges
Item# SW747 • 1,000 mcg 250 Lozenges
it's about $7.99
You might need supplementation of Magnesium and Potassium. Both get depleted easily during warm weather.