What Causes Redness On Skin After Performing Physical Activity?

Hi,

Im 26 years old and have been having these symptoms since I was 13. I have seen several Dr.'s, and none have actualy run tests and helped. My symptons are the following; Everytime I run, Jog and walk for about 5 blocks non stop my skin on my thighs start turning red and hurting if i continue the activities it starts spreading to the back of my thighs to my buttocks and my back. Its excrusiating pain. I have no sweeling, no rash, no bumps, no blotchiness. just even redness and pain. In the past I got on the vibraslim machine and withing 2 minutes into it I got the sypmtoms full blast all over my body. I Jumped off the machine and started crying because I couldnt bare the pain. this has never happend with any other machine not even a treadmill. I can do aerobics, and swimming without a problem. When I have Mentioned this to Dr.'s they would respond with oh its an allergy take benadryl and claritin. I've tried all allergy pills, I've tried aloe vera, creams everything. nothing prevents it and nothing makes it go away. I literally have to sit and not move and at all and it slowly goes away on its own. I hate this feeling. I'm a farely active person. I love to run and wish I could go on Marathons. I also wanted to become a police officer but it eems impossible to me with this condition since theres a lot of physical activity. Please HELP!!! I'm desperate.