Diacetyl - fermentation practice or infection

[41576nbk]

I brewed an ipa 3 weeks ago and transferred it to secondary after one week in primary (typically I allow 10-14 days in primary, but I was leaving out of town). Active fermentation was complete by this point and gravity went fro 1.055 to 1.012. (Primary temps were at 63deg for days 1-4 and 68deg days 4-7)

I went to add the dry hops today and can smell and taste a strong butter scotch aroma, which I think is diacetyl. Can you taste the difference between diacetyl caused by infection vs. fermentation schedule/temp? My guess is I took it off primary too soon, but would like to know how one could tell the difference.

Also, assuming this in not infection, can I make a new yeast starter and pitch that into the secondary to clean up the diacetyl?

Thanks!