I can be doing well and then a few days before and during my period I become depressed. Mine usually takes a week to wear off - hormones!

I've started to circle on my year planner when I start to feel rubbish, and have realised it's going to last only days. I actually feel better for knowing that and hopefully will be prepared for the next month!
Maybe write in your diary/calendar when you start feeling down and depressed, and see if there is a pattern. It might help to know that it only lasts a few days (or so). Exercise is supposed to help, so maybe go for a walk/jog or something else, which you enjoy doing. Reading may also help to take your mind off of things (not at the same time though, you might bump into something!

). <--- bad joke.
I know a really bad cold will not help matters though.

Remember though that your cold will not last long either, and it'll be a matter of days before you're back to feeling well again.
I hope you feel better soon,