What Causes Swollen Feet In Spring And Summer?

Hi my feet swell but usually only in the spring and summer. Could this be caused by flip flops it seems to only happen during the time that I wear these types of shoes? I have taken water pills and I think they helped slightly, however the swelling goes down after a week or so and than flares back up in a few days. I have gone to an Urgent Care and they gave me water pills as I said, but I wonder if it could be something else like allergy s since it only happens during the warmer seasons. We live in southern Ohio if that helps. Thank You for any help you can give me.