Probably because you got your Smithing to 100 at a low level. Your overall level increased, while your Heavy Armor/One-Handed (or Two Handed) stayed the same. You were fighting mobs a few levels higher than you if you take your combat skills into consideration.
yea, I spent the first couple of hourse power smithing, because I got obsessed with wearing Dragon Heavy Armor (Dovaakiin should wear the skin of his kind, right?)
but now I'm on level 33, my one-hand and heavy armor is around the 60s and I'm still getting owned by Elder dragons