Is it an english saddle or western? If it is western you most certainly can overtighten the cinch (I'm sure it's possible with an english saddle as well)but usually it causes problems where the girth rides not on the withers. Again, assuming it's a western saddle you may want to look at your pads for wear and degredation of quality.
Another point I would like to make is that your horse should never be sore after you ride it unless you put in an all-day (10+ hours in the saddle) hard ride. If the horse is sore after just average work there is more wrong than just being out of shape.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill