Pycharm awlays closes my new tab

All of a sudden I have been experiencing this problem: 

I opened a new file in pycharm; the file was showed in a tab nicely. Then I switched to another window, for example, a web browser or a window explorer to check other things. When I switched it back to Pycharm, all of the previously active tabs were gone! I had to use `ctrl + tab` to open again all the closed tabs. 

This is quite annoying, I have been using the professional edition of this tool. I even tried to uninstall and reinstall the latest version -- it still does not work. Whenever I click anywhere else and switch back to Pycharm again, I lose all the tabs!