My son actually has this. When the eye turns inward or outward it's called strabismus. The eye doctor told me it is inherited. I think the only way you could develop it is if you had a traumatic brain injury. And I'm not talking a bump on the head that gives you a lump, but a seriously bad injury that causes unconsciousness, coma, and neurological damage.
Our eyes naturally turn inward when we are looking at something close. This is called convergence. It's normal and you would see it if you were looking close in a mirror. When a person reads, their eyes converge to focus the words on the page.
If you want an expert to check it for you, go to a behavioral optometrist. They are like a regular optometrist with specialized training. For the cost of a regular eye exam they will look into it for you. You can find one in North America (not sure you're there, but if you are...) by searching www.covd.org
Also, even if you have it, it's not the end of the world. You can do vision therapy to fix it. You can read the book "Fixing My Gaze" about a woman who fixed this issue when she was 48. We did vision therapy for my son and it fixed it for him. No surgery, just eye exercises.