I know the fanfic that you are describing. I just forgot the title of it. It's on this site. The part you are talking about is where Sesshomaru and his teammates had to get a hair cut for speed. They are training for the Olympics.
The one you are describing is All or nothing by Forthright, rated T, and I am pretty sure it is on ff.net as well as here on Dokuga. But here is a link for anybody interested:
The plot does not sound at all like what Serena is describing, the cut-off hair is the only common feature.