It's perfectly normal for a south Texas lake, built during dry weather, to absorb lots of water, and drop a foot, even more. Water wicks into dry soils.
If the lake is leaking, you should see water on the backside of the dam. Walk below your dam, look for moist soils. I have seen a number of south Texas ponds and lakes where the dam sits on top of the ground, improperly cored. Those are the lakes which leak. Did your contractor core the dam?

Teach a man to grow fish...
He can teach to catch fish...