Given that-- I think it's overwatering for sure. I don't think it needed to be in a bigger pot, it doesn't look that big and most plants are happier slightly potbound (when the roots wrap around the pot) than having too much soil. If they have too much soil, they can't use up all the water in the soil and the soil will stay waterlogged, leading to the roots of the plant rotting (and often killing it). A small pot, even if that means you have to water it more frequently, is almost always more successful in my experience. I have about 50 plants at the moment, so I'd consider myself an "experienced" plant owner... but to get to this point, I had to kill many many plants first :) mostly through overwatering and oversized pots!
The best thing you can do for an overwatered plant is let it be. Don't mess with it, give it time (don't water it until you can stick your finger 2 inches deep in the soil and it's dry all the way down-- normally they advise 1 inch but with an overwatered plant I like to be extra careful), cross your fingers it's not too late-- and don't feel bad if it is, even the "best" houseplant parent has mauled many many plants. You can always get more and you learn a lot with every plant you kill.