Old abrasion turned into a discolored scar that looks wrong and sometimes itches. could it be an under the skin infection?

Unlikely: While it is conceivable to have an infection, it's very unlikely. In all likelihood, your scar is irritated and is changing colors do to that irritation. A trial of something like vaseline over the top of the scar for a week or two and trying to scratch it, will make a big difference. If you have further questions please see your doctor.