You should fully charge it on your balancing charger and then discharge it, either by running your gun off it or using a discharger. If it does it again then there's something wrong with it. If it's not more than a few months old, tell the retailer you want a refund, because it's not fit for purpose.
It's not a case of if a cell ever drops below 3V it is instantly and irrevocably fucked, it's a case of 3V is a convenient number to remember as a rule of thumb and the damage that occurs through over-discharge begins in tiny increments somewhere below that, but not at a set voltage, because it depends on the make, age, history of use, etc., and the lower the voltage drops, the worse the damage will be, but still not necessarily fit for the bin. LiPo batteries can swell without it meaning anything serious, but to be on the safe side, I would charge a swollen one on a fireproof surface away from anything close above just in case. Inside a metal biscuit tin would be the cheapest excellent solution, because although a LiPo doesn't actually need much outside oxygen to split and start burning, for the oxidation reaction to really get going, like full on fire spitting flaming nuggets of white hot burning metal like a chemistry lesson on crack would require considerable additional oxygen and a closed metal lid puts a total end to that possibility, even with a little hole to allow the wire through the tin side (because the fumes will fill the interior very quickly and their production causing positive pressure must prevent fresh air from inflowing).
Is
this your charger? Because it ought to rebalance your cells automatically if so.