Hello,
It can be due to cholinergic urticaria. It is a form of chronic hives that is caused by an increase in body temperature but can be idiopathic also. Take some antihistaminic like Benadryl or Claritin and apply some calamine lotion on the hives. If the symptoms persist then please get it examined from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
You know I'm re reading this and thinking that I focused on the wrong part. I know that no one else will ever tell you this but I used to have the same problem. Try letting your razor get real dull. Enough where it still cuts the hair but stops shaving a layer of your skin off with it. Use a men's shaving cream like barbosol w/ aloe. And rinse real well. Your break out is sweat or perfumes stinging because it's getting into all the little micro cuts from the razor because you have sensitive skin. St. Ives is pretty much the only lotion I'll use. I've heard Aveeno is good too. And slow the heck down when you shave too, lol!
Are you shaving the insides of your thighs too? And then getting sweaty? And then walking around rubbing the insides of your thighs together? And wearing super short shorts? And you have sensitive skin? Sounds to me like just normal sweaty irritation. That used to happen to me when I was younger and tried to wear micro shorts on hot humid days to walk to the mall.
Maybe don't shave so high? Definitely cut out any products with perfumes in them. Get a lotion with vitamin E in it. The St. Ives (find one that isn't smelly!!!!) line worked well for me. Try slightly longer shorts that are made for exercising and protect your legs from rubbing when you're walking or running long distances. Or wear something stretchy and breathable under a skirt. Or capri's.
What does the rash look like? Describe the rash or post a picture.
There are so many things it could be, starting with heat rash because yes you can get it on your legs, so posting a picture or describing it would help.
Also, this article might help.
http://walking.about.com/od/medhot/a/legrash.htm