Is my code my intellectual property or the University's?

I'm an undergraduate researcher. I wrote a script to solve a problem when I was working on our previous research project. I was unpaid at the time.

Now I am paid for my research and am updating my script (so that it runs better and is more easily accessible to my research group and easier for them to maintain when I leave.)

Who owns my code? Me or the school? Does it matter that I was unpaid (essentially doing the research for fun) at the time I initially wrote the code?

I'm fine with my research group using it in the future, but I want to be able to use it in the future as well when I move onto graduate school.

My current plan is to really build on it and make a really nice suite that would be applicable to both what we're doing now and what I would like to be doing in grad school, but I don't want to do it if I'm going to run into legal hot water.

(I go to Purdue.)