I'm from the Depression forum, but I saw your post & had to respond b/c I react the exact same way to Valium! I didn't even notice that it was a problem. I thought everything was going along just fine & that the Valium was helping me, but my family started telling me I would get so mean. It was not right away when I would take it; I think it was probably about the time that it started decreasing from the peak levels.
I checked with a few of my friends & they said they noticed the same thing but didn't know how to tell me. I don't know what caused it, but now I avoid it altogether. I take Serax instead & try not to take it every day just as a precaution.
Good luck to you. I'll check back to see if maybe another poster knows why this happens.