Almost passing out in hot busy area???

I go shopping once a week at the same store. It happen to always be hot and crowded there. I walk around and usually when im almost done I start getting a headache and nauseous it usually goes way if I stop and crouch down and take big breaths. But after I get up it comes back and today it got so bad my vision got blury and my hearing faded out and all I could hear is a mess of talking and a ringing in my ears. I felt like I was about to pass out and it took a few minutes to subside (and freak out the cashier). It's happened 2-3 times. Im really concerned for my baby and im gunna ask my doctor at the appointment in a few days. Any one have this happen or know what this is from??