I strongly believe that the leak is in your return line(s). I would have the returns pressure tested, if you know how to do it and can, you'll save quite a few bucks. If not, get a reliable company to do the test, and make the repair/ replacement. You did not specify what kind of pool you have, nor it's age.

In the future please include as much info on the pool as you can to get more and BETTER responses to your questions.