This presentation could be due to a trauma/foreign body/body wound or localized spot of skin infection (likely to be caused by bacteria). A check up, appropriate disinfection and possibly antibiotics (oral, topical or in a shampoo formulation) are needed to solve the problem. Disinfection can eventually be started at home with very diluted hibiscrub or betadine or salted water baths. Buster collar may be useful as well to avoid any licking/scratching.
Please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.