weakness/weird feeling only when shopping

often when I go shopping, whether it be the mall, or walmart, etc. I get this feeling of being drained. I get tired, sometimes my neck hurts, and I feel like I sort of have a headache, and my eyes are tired. I just feel, so out of it. I feel better once I finally leave, and usually something like diet coke will make me feel better right away.

I'm sure it isn't a caffeine addiction. I get enough sleep. I never feel like this if I'm at some sort of function, hanging out with my friends at night somewhere, etc etc. when I exercise I feel fine. only during shopping I feel like this.

maybe it's because I dread shopping? I don't understand at all. my mom once said they recirculate air in stores? could that be it? or maybe some sort of irritant/allergy located within the stores or malls?