Did you use water in the extraction?
You will want to go back and do a liquid-liquid extraction to clear out impurities.
I suspect that you may have accidentally gotten an insoluble salt from fertilizer residue in the buds, possibly reacting with salt in the butter itself- normally this is not a problem at all, since most of us do butter in a melted butter/water/plant matter combination, and the ionic substances actually go into solution in the aqueous phase and are washed away.

Seriously, read the washed cannabutter thread. You can still perform a wash on your butter at this stage, no problem at all!