You have an extra sensitive sense of smell. Many Migraine sufferers have this phenomenon.
I can smell it too...but I never had migraine...I must say that the smell of the rains is the best sweet, relaxing, pure and it only came just for a a minutes...and I really love it...
Rain is usually accompanied by low atmospheric pressure. That allows air from the ground to rise up to your nostrils and so you smell the ground. You interpret that as the smell of rain.
I think it just smells a bit like tap water for some reason actually :S
Because God made rain.