The sand will be somewhat soft for a while, but it shouldn't feel like there's a bubble under the liner. To make sure the pool's not leaking, do the bucket test as mentioned above. Get a bucket and fill it partially with water and set it on the top or second step of the ladder going in to your pool (depending on where the water level's at). Make sure you have enough water in the bucket so that the waterline in the bucket is a three or four inches above the waterline of the pool. Now take a marker and carefully mark both waterlines on the side of the bucket. Give it a day or so and then compare how much the two waterlines have dropped. If they've dropped the same amount, then no leak! If the pool waterline has dropped more than the bucket, you've got a leak somewhere.
My only other thought might be groundwater pooling in the sand, but that's about all I can think that would cause bubbles under the liner.