There are multiple cells within a given battery and, while it's hard to tell without seeing it, it sounds as if you probably have a single bad cell, or maybe two. When they fail, LiPO batteries tend to fail pretty spectacularly, so I'd recommend you contact DJI and see if they feel it's worth an exchange on the grounds of being better safe than sorry (assuming it's still eligable). Shipping suspect LiPOs also isn't something to be done lightly and they may prefer to just send you a replacement and let you dispose of that one, so it's definitely worth a shot.
Personally, I don't take any chances with my LiPOs, but YMMV if it's a as minor as you imply. Even so, I'd definitely be using that battery as a last resort, being very careful about where I keep it, and closely monitoring the situation - if the bulge continues to grow, then I'd seriously recommend replacement and safe disposal, even if it means buying a new battery yourself. Keep in mind that batteries tend to fail because of shorts - which can be caused by the cell expansion you are describing, or by thermal expansion when under load, e.g. when it's in the air. Is it *really* worth the risk?