The lump you're describing could be something that is often referred to as a "King Bump". It's caused by the difference in box support between the edges of the King boxes and the centers. Since kings (Or California King) mattresses use two edge reinforced boxes the support is naturally sturdier where the boxes meet in the middle of the bed than it is halfway between the middle and the edge. I'd be willing to bet that if you looked the "lump" coincides with the point where the two boxes meet.
You have several options here:
1) Sealy's warranty does cover this problem if it has created a difference in bed height greater than 1.5" from the highest point of the lump to the lowest (non-button) point of your deepest impression. Sealy's warranty is 10 years non-prorated, non-fixit. (You should more or less get a comparable mattress or even the money you paid towards another mattress.
2) If you're set on getting a new mattress a change in your bed can actually help prevent the problem in the future. Platform beds are types of beds that are designed to be used without boxes. So, when you go to purchase your new mattress you will only buy the mattress and not the set. Since you're not using another manufacturer's boxes (you're using no boxes at all) your warranty will still be good.
3) If you don't want to do either of the above options very good quality memory-foam mattresses are less susceptible (not impervious) to this problem.