I always feel a bit unmotivated and "blah" after a 100-miler (I usually do about 1 per month from May-Oct)...it's a normal feeling for me. A 100-miler is a big deal (at least for me it is) and even after 3 solid years of riding and doing many centuries, I still find them to be extremely challenging. It is perfectly normal for me to NOT want to get on my bike (except for maybe a casual recovery ride) for several days after completing my "monthly" century ride.