Could you provide pictures? From the description I would guess root rot. If this is the case:

1. Check to make sure that your planter has proper drainage. Does it have a drain hole? Is it being obstructed?
2. Make sure your soil has proper air flow. Grab a chopstick or stick and poke your dirt to make sure its getting proper airflow so that the water dries out properly.
3. Make sure that your soil is pretty dry before you water it again. Monsteras don’t mind dry soil