Leaves curling down

I know you say it can't be overwatering, but It's overwatered. Or, the roots are rotting. Take a look at the roots. If they smell and look rough(dark brown, mushy, etc.), that's the problem. Cut all the rotted roots away and get the swampy soil out of your pot and replace it with fresh soil. Then, don't water it for a few days and let it dry out. If it is overwatering and not root rot, Just let her dry out completely and then water until some starts running out of the drainage holes of the pot. Good luck.