I had a similar situation after I gravel vacced my well established tank and the conclusion I came to was that I had disturbed the fertilizer tabs (not from the Co-Op) I had placed in the substrate and released ammonia/nitrite into the water column. It was briefly at 10ppm and I had to do 50% water changes for a few days in a row before the I was able to get out in front of it. Had a bacterial bloom as well which I assume was the tank trying to also deal with the excess nitrites. Haven't used the gravel vac since and actually added additional fert tabs but no subsequent spike.