I had a bikini incision. It healed but now hurts after about a year. Why is that?

Nerve damage. Whenever you have any operation, it is unavoidable to cut nerves in the skin, even for small incisions. As the nerves try to grow back, they encounter the scar, and can form a neuroma. Speak to your doctor, you may benefit from a trigger point injection in the area and gain some relief.