I am concerned that this is a bacterial infection which is needing attention. Considering its size then this needs to be seen by your doctor, or the ER over the weekend owing to its rapid growth. It may need draining, and certainly appropraite antibiotics.
A faruncle is the name given to an infected hair follicle which develops a Staph aureus infection. It can lead to abcess under the skin, an abcess is an area of infection 'walled off' which makes it harder to treat. It could be a fluid filled cyst, but one which grows this quickly is usually infected.
Depending upon its location, it could also be an infected gland or lymph node. Again a doctor is the first port of call.
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