Re floating liner in a pond by a lake, when you were digging did the hole fill with water and was the lake lower than now?
You may have a leak but the other common causes of liner ballooning are a high water table and run off getting under the liner, there are two methods of addressing those causes,
1) make the pond semi raised so that the pond level is above the local surrounding ground
2) improve the drainage around the pond.

Both of these have the same effect, which is to raise the pond level above the 'water table', meaning, that at any depth the pressure of the water inside the liner is greater that than the pressure of the water outside the liner at the same level. That in turn means the liner is always pushed outwards against the soil. If you did this and it is a leak you would see the water level in the pond drop.
Did you leave a hose under the liner? If not can you put one under, say 1/2" garden hose, and set up a slow syphon to remove the water under the liner, it needs to be slow otherwise it may 'suck' too quickly and cause the liner around the hose to temporarily seal to the soil thus starving and breaking the syphon. As you syphon out the water from under the liner the pond level will drop until the liner is fully 'inflated' and against the soil so I would suggest you simultaneously fill the pond to keep the level up.