Hello Good evening,
Since your symptoms of acute onset and associated with tenderness, So more chances it is a furuncle
or boil, which is due to infection of hair follicle or sebaceous gland
caused by some types of bacteria, nothing to worry so hard.
If you are able to see it's color then it should be seen red if inflamed or white if some pus is in there.
Please take some analgesics which also cause as anti-inflammatory, take antibiotics as per instructions of your GP if some pus is there.
Hopefully it will be subsided by analgesics and antibiotics.