Why is only one of my lower eyelids swollen?

Maybe it's from stress. I know it sounds weird, but when I get really stressed out my eye swells up a little on the corners and it looks puffy and retarded. It's normally just one eyelid and the eye itself is fine. I recommend getting a towel rag and putting hot water on it and placing it on the affected area. This is what my optometrist told me to do when I get them. It seems to help for the most part. I hope it helps and isn't anything serious.