I know this is an old post, but I just wanted to report that the exact same thing happened to me. My plant is in its second year in the same pot, growing on my balcony in DC. It had been rainey and damp through April and May and even though my balcony is covered, the soil remained damp all of the time so I hesitated to even water it, like ever. Oh yea, and the leaves were withering big time.
I managed to lean the pot on its side and drill a bunch of new holes and out came all the water. I had checked the bottom of the pot before and it looked like there were enough holes but I guess not. Maybe the original holes became clogged as this pot/soil sat on the balcony all winter.
Anyway glad I found this post with all the good advice. I'm hoping to be able to transplant this root into the ground next off season.