Drop In Pool Bulge

[RayZ]

We have a Viking fiberglass kidney shaped drop-in pool. It is supposed to take 47,000 PSI. We just noticed a 'tumor' on the bottom of the pool. Approx 4'x1'x2-3" high bulge. It sounds hollow compared to the rest of the pool bottom when you tap on it. We have had a considerable amount of rain this season so although the ground is saturated, the water has relief so as to not build up pressure as far as we can tell. We have the installer coming today for a look see but they have already told us they have never heard of this problem before. We wanted your opinion before he got here. What say you?
Thx - RayZ