6 or 7 times over the past 3 months my lip, usually the bottom, will swell for no apparent reason.  After the first few times I tried to pay attention to food intake and activities to find a common denominator, but have been unable to do so.  This happens most often in the early evening, after dinner, but happened the first time in the middle of the night and at least once during the day.  The swelling is sometimes on just one side, or starts on one side and moves across until the entire lip is swollen.  The swelling last for several hours, then gradually fades.  I cannot find an insect bite, can't nail down eating a particular food, have had a cat for 7 years and never had a problem until about 3 months ago.  I haven't changed lotions, shampoos, other cosmetics, toothpaste.  It's a big mystery.