Im 23 years old. 3 months back I noticed a small bump on my scalp which I though would go away in a few days or weeks. But it didn't. Its been 3 months and the bump is still there, it is not painful at all. And it doesn't grow. What is it?

You need to ask your doctor. Nobody on an online help forum can diagnose something like this without even seeing you.

There's really no way to determine that online. It could be an ingrown hair, a mole, a cut, a pimple, cancer, anything. But we can't determine that here. You'll have to see a doctor for a diagnosis.