I studied mathematics at the undergraduate level, but I was always interested in computers and I spent my days as an apprentice at a super-computing center. Then I studied a mix of computer science and mathematics for my PhD at a computer science school. Nowadays I am a sort of amphibian: I work in a math department, my colleagues think I am a computer scientist, and my interests span from programming languages to homotopy type theory.
I can recommend doing math as a major at the undergraduate level first, and then switching to computer science later. It's not that hard to do, and having good general background in mathematics is a huge advantage in theoretical computer science. You never know when someone is going to come up with elliptic curves in cryptography, or homotopy theory in type theory, or advanced linear algebra in data mining, or Fourier transforms in video encoding, etc. I never heard any computer scientist say that they knew too much math. The math you learn as an undergraduate will stay with you. It will be much more difficult to learn new math later in your life.
While in college, you can still do lots of interesting things in computer science. Here are some from my area, hopefully someone else will suggest ideas from other areas. You can read computer-sciency mathematics on the side; you can participate in open-source projects, possibly ones that involve a lot of mathematics, such as formalization of mathematics with proof assistants; you can learn mathematically inspired programming languages; or the mathematical theory behind them. This way you might find out what interests you.