Don't go jumping to conclusions too fast. It could be a thousand things. A few years ago, I found a small lump on one of mine so I went to the doctors. He said that only about 1 in 100 patients he sees for things like that turns out to be cancer. In my case, it wasn't. When you say they are tender, are they both tender? If they are, then I doubt it is because it would really only affect one at a time. If it is just the one, then my guess is that something hit it and you just never realized it.
Make an appointment for the doctor. In the meantime, don't freak out. Chances are it is nothing.
Best wishes.