Be sure and dump your winnings from the fights into a chest somewhere (I put mine in the one near the archery range at the camp).
Every time you win a fight you receive a piece of armor which adds up quickly, and there is an over-encumbrance feature added but only seems to affect you when fighting.
Its represented by an icon of a man carrying what looks like a bookshelf on his back, it shows up to the left of the health bar.
So far I have experienced two stages at first I can move just more sluggishly, and when more weight is added during combat you cannot move you feet at all.
So the next time this happens to you be sure to check your inventory and see what you current carried weight is.