The most likely cause is that you fought a jelly (a red capital J). Hitting a jelly can cause your weapon to degrade, while being attacked by a jelly can cause your armour to degrade. In both cases there is a message that displays, but that is easy to miss.
The best way to fight an early jelly is stones, assuming you don't have a ranged magical attack. Stones are always lying around the first levels and throwing them is effective, even if you don't have a sling. Also, stones are one of the few items that the jelly will not eat - and every time it eats, it heals itself, so shooting a jelly with arrows or bolts can actually make the fight more difficult, as the jelly will split into two jellies once it has eaten enough items.
As the majority of the races are faster than a jelly, you can run away from them and pelt them with rocks from afar.