If it stays swollen or swells during or after charging,
dispose of it.
If it swells slightly during a run, but returns to shape and doesn't have soft spots after the pack cools, it *should* be safe to continue using, but I, personally, would be shopping for a replacement pretty soon.
If possible, and if you're not already, store the pack in a fireproof container [metal ammo can would be perfect] well away from anything flammable.
Take this info as you see fit. I know batteries represent a high expense to a lot of us, but if I had a pack swelling [haven't had it happen to me yet], I would dispose of and replace it.