Regular egg then soft-shelled egg 13 hours later

[My Three Chicks]

Hi. I have a healthy 16 month old Speckled Sussex who lays 4-5 times a week. She laid a regular egg this morning at 9am. (Although it was almost white instead of her very light brown which she'll do on occasion). Then tonight I was checking their coop cam before going to bed and I couldn't believe my eyes so I rushed out to see...she had laid another egg (sometime around 10pm). It was soft-shelled. See pic.
This was very odd and it's never happened. Should I be concerned? I thought chickens couldn't lay more than once a day. Maybe her developing egg just fell out for some reason?
FYI they also always have a bowl of oyster shells and baked eggshells at all times.
(I know it was her's for sure. I only have 4 girls and they all laid today. And the soft-shelled egg was where she was roosting alone).

 

[Callender Girl]

I guess I don't get too concerned about a single soft-shelled egg, especially if the hen has been otherwise producing normally. I let all my girls have access to oyster shell, and I figure once in a while, there's just a glitch on the production line.

Hope your Speckled is okay and gets back on track.