You may have a tumor.
You tell me, you are the Doctor.
That happens to me so often that sometimes I avoid eating oranges. I suspect it's something in the citric acid.
Could be the acidity if u scuffed your skin abit
was the orange sweet ? they're using chemicals to artificially ripen them .
the flesh of an orange is where flesh eating bacteria come from. You didnt know?
Finger nails can be fragile......
This is a really late answer, but I want to clarify for anyone who reads this in the future. It is the citric acid. This same thing happened to me yesterday. The pain wore off after awhile. The skin under your nail is very soft and sensitive and once the citric acid touches that skin is causes immense pain, but it lasts no longer than a couple of hours. There is nothing to worry about, but next time when you peel an orange do not use your fingernail, use the sides of your thumbs to peel it.