When it is swollen, can you feel a hard knot?
Does the lid turn red? Is is sore to the touch?
I would say that it is only as red and sore as you would expect tissue to be when it is very swollen. It is not hot and very sore as I would think would come with infection. There is a mild redness and soreness only.
It may be an allergy that causes the lid to swell. You would have the itching and drainage. And it would clear up quickly.
Another possibility is that it is related to the sinus connection. In some people, there is a connection from the sinus passage to the eye socket. Some people can blow their nose and cause their eye to bulge out. Sneezing or blowing nose or sinus infection can cause the eyelid to swell up like that. This would just be air in the tissue and would feel soft. There is no treatment necessary for this.
Finally, rarely some have an enlarged vein in their eyesocket (called orbital varix). Straining can cause the vein to enlarge temporarily. This can cause the lid to swell or the eye to bulge out.
You should have an eye doctor look next time it happens.