Encumbrance

Facts
There are six levels of encumbrance in Nethack: Unencumbered, Burdened, Stressed, Strained, Overtaxed and Overloaded. The exact inventory weight which determines which of these levels you experience is based on Strength and Constitution.

The maxmimum carrying capacity without becoming burdened is 1000 weight units. A player can carry up to 3000 weight units before becoming Overloaded, at which point any kind of action except dropping items becomes impossible.

Strategy
Your encumbrance level can be reduced by:
 * Using a bag of holding
 * Gaining strength or constitution, including polymorphing into a stronger creature.
 * Dropping items

Commentary
When being suddenly polymorphed into a small creature, it is common to become instantly overloaded, and unable to move.