Welcome to the forum!
Where are you located? If it got well down in the 40s, or lower, this might be the peppers reacting to those night time lows.
Fortunately, I don't get any diseases on my peppers, even in the very wet times, when many other vegetables get fungal diseases. However, there are a number of leaf spot diseases out there - bacterial, fungal, and viral. This does not look like any that I've seen pictures of, but those pictures are always of the advanced stages of the diseases. Keep an eye on them, to see if it gets worse - as long as it doesn't start increasing in size and numbers, and the peppers are doing well otherwise, I wouldn't worry about it, though if you want you could spray some sort of safe fungal spray, like baking soda or potassium bicarbonate, or some Serenade, though you don't really know if it is a fungus, if it is a disease.