I would agree that it's the lack of drainage on your soil... No drainage no oxygen.. Root rot everytime! Definitely transplant into a bigger pot... I would suggest at least a 3 gallon depending on how big you want the plant to grow. If you need to buy a pot I would suggest buying a smart pot. When you water you want to be sure any excess water is draining from the pot rather than drowning the root system. I personally wouldn't water again until after you've transplanted but once you do, lift your pot once watered and note how heavy it is... Then wait a day and check it by lifting the pot, if it still feels heavy give it another day and check it. Repeat until the pot starts to become light. Correct me if I'm wrong but that's what I go by
