It's not clear to me if you were asked by your boss to write the app, or if you just saw the need and wrote it on your own.
It's best if you can think through your position before starting work rather than afterwards. However, at least you are thinking it through now so you are learning.
If you saw the need and wrote the application, ask for money. It does not cost anything to ask and you will learn about negotiating. Make a case for being paid extra and put it to your boss. Give it everything you've got. Then if he says no, give the work over with a good attitude and tell him you are glad you can participate and be helpful.
Remember that everything that you do builds your reputation, so if you are doing extra work and fixing problems that are beyond your formal responsibilities while at the same time meeting your responsibilities and not stepping on any toes, you are actually ahead of the game. At some point if the job is not giving you what you want, look to move on. Then in your interviews, you can speak in positive terms about the extra contributions you made.