Yes, rotting from being too wet, too long.
Water only when the top 1cm is dry before watering again and she should recover...... hopefully.
She will need transplanting soon, so once she has recovered and making good new growth, this will be an ideal time to do so.
Once in the new pot, it is best to water deeply to run off every few days and not watering again until the top 2-3cm (note the change) of the soil has dried out. This could mean only watering every 2-7 days, depending on temps, the plants requirements and the water holding capacity of your soil mix.
Hope this helps, Organoman.