Irregular Eggs

I went out to gather eggs this noon and look what I found! The green egg is from Crocus the Easter Egger; the tan egg is from Squill the Speckled Sussex; and the large brown egg is from Thelma the Delaware-Buckeye hybrid. But who in the world laid the tiny egg? It's about the size of a walnut. I might think it came from a bantam hen, except that I don't have bantams. The dark brown shell is the same as Thelma's eggs. Can she have laid an egg-and-a-half today? Is that even possible? She's been working hard lately, clearly making up for my older hens who lay only twice a week. Strange....