It could be a bad cell. You'll know for sure if that cell is continuously lower than the others when you discharge the pack. Swelling is obviously not a good sign either. There is a chance that cell just happened to get more discharged than the others, but will be fine once you get the pack balanced.
Due to the design of a balance charger, it is often very time consuming to get one very different cell voltage to match the others. In this case what I do is rig up a charger lead with two pins at the end. I use two pin headers like the ones you solder on to an FC to connect servo connectors. Then I use the pins to tap into the balance lead at the pins that correspond to the low cell. Then I use the charger set to 1s to charge only that cell up to the voltage of the other cells. Make sure to constantly monitor the voltage so you don't overshoot the other cells.
