The swelling is either the result of a re-bleed into the lip with a hematoma formation or, more likely, residual scar tissue that often build up whenever a through and through laceration occurs. If this persists after several months, minor surgery may required to correct the problem.
I have a similiar situation. I had a car accident many years back and have excess scar tissue as a result. I also had 15 stitches inside my upper lip. Years gone by and it is noticeably larger. I am going this afternoon to have the scar tissue removed via laser. My lip wasnt bad for the first few years but over time it became alot worse. I will let you know the results if you are considering having it removed. Best of luck~Katie