The price a bond is inversely related to the interest rates of newly issued bonds being offered in the market.  As the interest rate goes up for newly issued bonds then the price of your bond with the lower interest rate is going to fall.  It is common sense, an investor is going to want the bond with the higher interest rate so they will pay less for your outstanding bond with the lower interest rate.

The equation to calculate what the price of the bond in the secondary market will be is:

PRICE OF BOND = 1/CURRENT INTEREST RATE