M/T suddenly notchy

my '08 m3 just passed 13k miles and it seems like the shifts have gotten particularly notchy. not sure if it could be the cold weather we've been having lately, though it didn't seem act like this through the first half of the winter. it became particularly noticeable one afternoon driving home from work when i went to shift from 3rd to 4th with just a tap as i occasionally do, and almost ground gears as it didn't shift smoothly. if it's normal behavior i can get used to the difference, it just concerned me because it seemed to be a pretty noticeable and sudden change. any ideas what this could be?