Muffin-top waterbed

I have a king size waterbed, single bladder, anti-wave batts, with soft sides. Recently, I thought that it seemed to have less water, even though I couldn't find a leak. Thinking that it might have a small leak under the bladder, or at least some place I hadn't spotted, I drained the mattress and looked for puddles. None found. To be sure, I inflated the mattress with air and checked for an air leak -- none found.
I then replaced the mattress and refilled. There is now a rim around the edge of the mattress, about 8 inches wide, that is depressed an inch or two. The water fill is near the minimum, just enough to not bottom out. The "moat" is making the bed less comfortable. What caused this and what can I do? One thought is that I might have some wrinkles on the bottom, so that the mattress doesn't stretch out to the sides as it should, although I would think that would make the spacing uneven. The other thought is that when I inflated the mattress, I stretched the vinyl in a way that causes the deformation. If so, I suspect I need to get a new bladder.
Any ideas?
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