You should use a profiler to locate your memory leak.
In NetBeans, at the top next to "Run Project" and "Debug Project" buttons is a "Profile Project" button (Alt-F2).
First run, it will may ask you to calibrate or something.
Afterwars, you can choose to analyse CPU or Memory. If you click on Memory, check "Simple," and click Run, you can run your project and see what's using memory.