It's possible that your employer treated the income that you earned during that month when you were commuting back and forth as taxable in both states - which would be correct. Income is taxed in both the state in which you live and the state in which you work. The income that you earned for that month is taxable in both North Carolina and Virginia.
However, you would qualify for a credit in Virginia for the tax paid in North Carolina on the income that was taxed in both states.
