Is this a Eureka lemon tree by chance? As it's common for these types especially
This usually happens when a little bud mite sucks the sap from the lemon flower, causing the flower to have a deformed ovary. It's still edible though of course!
This makes sense actually because the auxins are most high in the ovary & is being exposed to the membrane allowing it to become elongated.
Ahh yes, of course