What Causes Swelling Under The Left Eye?

For the last couple of weeks I've had a swelling under my left eye. It looks like a large water blister, and seems to get less noticeable as the day progresses. It isn't discolored and doesn't hurt. I noticed it after my annual eye examination but don't know if there's any connection. I always sleep on my left side. I welcome any thoughts or suggestions you may have. Thank you.