Absolutely! I've been pretty happy lately but I won't pretend that I don't have my bad days like everyone else. Its ok to have a bad day. Just don't setup camp and live there! Life gets stressful. Learn your lessons and move on to better days!
Sometimes when we have a bad day it would be nice if we could just pull out a box of all our favorite things that make us happy so we can smile again. :) I got the idea to create a "happy box" one summer before I moved out to my own place so I would be prepared for living on my own in case the inevitable bad day ever struck.