5.7x28 and 5.56x45 NATO are two completely different cartridges. 5.7x28 is a handgun cartridge primarily found in the FN FiveSeven pistol, P90 Submachine gun, and PS90 semi-auto carbine.
5.56x45, on the other hand, is a rifle cartridge and is usually found in guns like the M16/AR-15 series of rifles, HK-33, FAMAS, L85A1, and Galil.
A 5.56x45 cartridge is unlikely to even fit in the magazine, much less chamber or fire, in your pistol. The only large company currently making 5.7x28 ammunition is FN though there are a few smaller boutique makers like Elite Ammunition that offer it.
Not to be condescending or preachy, but you should really be more careful when buying ammunition. You're really quite lucky in this case that all you're out is the cost of ammunition that you cannot use (unless of course you buy another gun or trade the ammo for the correct cartridge). There are, however, firearms which can chamber and fire the wrong ammunition with potentially dangerous results. The general rule of thumb is to fire only the caliber which is marked on the firearm though there are a few exceptions such as .357 Magnum and .44 Magnum revolvers which can safely fire .38 Special and .44 Special ammunition respectively. If you have doubts, most firearm manuals are fairly specific about what type of ammunition should be used. The thought that "it fits OK so it must be alright to shoot" is a very dangerous assumption.
