Sick as a dog!

So, normally I would not want to wait this long to make a post, but I have just been miserable. Sunday I started getting a headache. One of those that starts at the back of your head and moves forward over your eyes so that you can barely open your eyes. I could not sleep that night and it continued all day Monday. Add to that food poisoning from lunch at Arby's and I was in great shape the whole day.
Today, I woke up feeling fine, but it got me to thinking about something. What in the world does "sick as a dog" mean anyway? I have had dogs all my life and I have never noticed them be particularly miserable when they have been sick. Actually, I can't really remember them being sick very often. How do these sayings start anyway? Like "the bee's knees", what's that about? Is it just because it rhymes and sounds cute? Someone who used to work for me had a real thing about that saying. She'd get very irritated every time someone said it. I didn't think it was that bad, but she would go bonkers when she heard it. I guess everybody has something. I don't have much to say on this subject (I know surprising!), but I just want to know who comes up with these things?
Here is something that made me giggle today, though.

Your Bumper Sticker Should Be

Barbie is not a slut - her legs won't open

What Bumper Sticker Should Be On Your Car?