A small white dot underneath the skin that is painful when pressed could be due to various reasons. Some possibilities include:
Cyst: A small, fluid-filled sac can form underneath the skin, causing a white appearance and tenderness when touched.
Ingrown Hair: Sometimes, when a hair grows back into the skin instead of outward, it can cause a small bump that may appear white and be painful.
Wart: Certain types of warts can have a white appearance and can be tender if pressed.
Foreign Body: It's possible that a small foreign object, like a splinter or a tiny piece of glass, got lodged under your skin, causing irritation and pain.
Pimple or Pustule: An inflamed pore or a minor infection can lead to a white bump that is sensitive to pressure.
Ganglion Cyst: These are non-cancerous lumps filled with a jelly-like substance, typically found near joints or tendons.
Since I am not a doctor, I strongly recommend consulting a healthcare professional to examine the spot on your left index finger properly. They can provide a definitive diagnosis and recommend the appropriate treatment, if necessary. They may perform a physical examination, ask about your medical history, and, if required, order further tests to identify the underlying cause. Always prioritize your health and get professional medical advice when experiencing any unusual symptoms.