EXTREME ITCHING when I exert myself, get hot, blush

I'm 16 years old and for as long as I can remember I always get itchy when I first start to sweat.  In an uncomfortable way.  But since about when I turned 15 whenever I exert myself, get hot for any reason or blush I get incredibly itchy.  I itch on my face, scalp, neck, my chest, back...  all over my upper body really.   I have never once itched from the waist down which to me seems extremely weird.  I'd love to go to the doctor but we don't have enough money to pay for the co pay thing so I'm just trying to figure out what is going on with me on my own.  It's almost debilitating having this much itching every time I start to get a little warm.  And I'd love to stop it some how.  

So far I've gone to the nurse at school a few times randomly to see what they think.  They have only given me some benadryl which has done absolutely nothing except make me a little drowsy.  

But back to being itchy.  Either I get hot, exert myself (football/basketball practice) or just blush and holy hell do I get itchy.  It is incredible how itchy I get.  It has lasted at most 3 minutes or so.  But usually I'm normal temp with no sweating or anything and then I get incredibly itchy and then I might start to sweat a little bit.  But the second I actually start sweating the itching is gone.  And I've done little experiments with this.  I was inside an air conditioned house and then I quickly put on a crap load of clothes and then went outside (back yard of course lol) to 80+ degree temperature.  Within 10 seconds of going outside I was incredibly itchy once again but it lasted literally 20 seconds if not less and then it was gone.

So from my point of view it looks like it's something irritating my sweat glands or something.  

My skin isn't dry, I have no known allergies, no dandruff, eat healthy, exercise 5 days per week and drink tons of water and gatorade (about 3:1)  

If anyone can help me out you would be my hero because having to live with something like this is just horrible.  But I guess if I can fix it I'll just have to always be sweating so I don't have to go through the transition any more lol.  Thanks.