What can be causing pain in labia major?

Local, referred CMD: Local causes include infections like herpes, other stds, hernia (women get them too), erosions, cysts, lymph node, rarely tumors. Referred pain can be from, e.g. Passing a ureteral stone, nerve irritation, etc. Need an exam.

Many sources pain: There are many potential causes for pain in the labia majora which may be due to any component of the majora including the hair follicles, oil glands, fat, and abdominal wall with problems due to infection, cysts, vascular malformations, hernia, tumors, etc. See your gynecologist for an examination to discuss possible causes.