Welcome to HCM.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.
Based on the information provided it appears as you have a minor injury
may be small bruise
while swimming that you are unaware off. It will heal on its on in two to three days.
Other possibility is that, it might be some insect bite, but not associated with itching it is less likely.
I advice you to wash the area with plenty of water and mild soap, apply moisturizer. You can consider antihistamine cream in case it starts itches.
Chicken fox is very unlikely.
I advice you to see physician in case if it persists or the severity increases.
Hope I have provided the information you needed.
Wish you good health. Take care.