So i've been riding road for only a week or two and after 7 miles or so my butt starts killing me. it's actually more the bottom of my butt where it contacts the seat. by 13-14 miles the pain is definately my limiting factor, i need to fix this. i'm riding a stock trek 5200 with pearl ultrasensor shorts. any suggestions? if it's get a new saddle then which one? thx