Well ... 68 is a bit too warm to pitch a kolsch. I'd pitch no warmer than 58 and keep fermentation in the high 50s/low 60s. Like I said, you need to approximate closer to a lager pitch for these cool fermenting ales, if you want to brew them right. Also, if you don't pitch enough yeast it won't be quite as clean as it would be had you pitched an appropriate size starter (and a swollen smack pack is an adequate pitch for about a 1.040 ale, assuming the yeast is near 100% viability - which it never is. And underpitching can cause diacetyl.)
And, if you pitched at 68 and then forced the fermentation to cool down to the low 60s that very well could cause the yeast to throw diacetyl, as this is one of the classic causes of diacetyl.
Bottom like is, yeah, you might want to do a d-rest just to be sure. Next time I recommend making the right size starter (check the pitching calc at www.mrmalty.com
and use the "hybrid" pull down menu) and pitch much cooler. The idea is to picth the yeast and let their metabolic activity raise the temp to the correct range, so for best bet pitch 4-6 degrees below your targeted fermentation temp. especially for beer like kolsch, which should have no yeasty off flavors.