Odd indeed. Either your SWG stopped producing FC and/or you had a small algae bloom in the works. Either way, the Rx is more bleach. SLAM away! Afterwards, do some testing to see if your SWG is operating properly.
Very good. Stick with it until you pass all 3 criteria. Make sure to look all around the pool for any potential hiding spots where algae can grow. Maybe behind a light or something? The rain itself shouldn't be a factor, but perhaps the rain picked-up and deposited some unusually heavy particulate in the air? The falling CYA "might" have resulted in less SWG efficiency and FC retention - maybe. Your water is not too warm, so that may be a stretch. Either way, knock it out now and start the new year on the right foot.