Why did this explosion make me fat? (A land mine increased my weight)

Solution 1:

It did not increase your weight; it gave you wounded legs, which decreases your carrying capacity. You'll get better after a few turns.

Solution 2:

Wounded legs is an intrinsic property generally considered undesirable. With wounded legs, your dexterity is reduced by one and abused every five turns. You are unable to jump, kick, or ride anything, and you become significantly more burdened. However, it is temporary, lasting usually only a few turns. After it is gone, your dexterity returns to its normal level and your encumbrance returns to its previous level.

Legs may be wounded by:

• An exploding land mine

From the Nethack wiki.