Please help - Building pond - Water under liner

Hello.

I have dug a pond in my back yard, and am having some problems with it
- I could use any advice you may have. Here are the details:

The back yard is on a slight slope, and there are other yards from our
neighbor's homes that are at slightly higher ground than our yard. I
dug the hole approximately 12' by 12' by 3' deep, and had to build a
slight retaining wall on the lower side of the pond so that it would
be level all around. I put the liner in, and filled the bottom of the
pond with river rock and slightly larger stones (about 3" round).

The major problem that I am having is that there is water buildup
underneath the liner. The way I discovered this is because I put the
liner in the hole, and covered the bottom of the pond with rocks, but
did not fill the pond with water. A day later, the liner of the pond
had bubbled up because of water having collected under the liner!!!

Where did the water come from? Who knows! There has been no rain for
days, and we are in the middle of summer, so it has been hot. My
guess is that people have been watering their yards, and I have water
from the ground collecting where I have dug my pond hole. The strange
thing is that my yard is not normally wet, and my grass is quite dry,
which tells me that the water is collecting underneath the ground.

There is no hole in my liner, as I am not getting any water come above
my liner from underneath.

Does anyone have any ideas on what I can do to deal with this problem?
What if I were to just fill the pond, in the hopes that the water
under the liner would just dissipate back into the ground. One more
thing, the ground is a very hard clay, and is not very soft or easy to
dig.

If I dug another hole at a level lower than my pond, and filled it in
with rocks, is there any chance that the water might migrate to this
other hole?

I am looking for an inexpensive solution, if at all possible.

Thanks in advance,

Faruk