could be bad gas. during the winter, some states see oxygenated fuel. i believe that drops the octane of the fuel.
you could also have an artificially high compression ratio from carbon build up in the chambers, or a piece of crud in the chamber preigniting the mixture.
if this just happened and all else has been the same. run through the fuel and try again. you may want to check your fuel pressure at WOT (where you hear detonation/preignition). make sure you are not losing pressure/ leaning out.