Hello,
Thank you for asking at HCM.
The armpit is an area that is wet due to sweat a lot of times and this causes good place for microbes to grow and get infections.
If it is painful It very likely is a bacterial infection but on the other hand the are there is very prone to yeast infection. The treatment for bacterial infection might make yeast infection worse so to be safe I would recommend:
1. Keep a good hygiene of the area, wash it with soap daily and gently dry with towel.
2. Wear cotton shirts and change them as needed if you have sweated a lot
3. use a atifungal cream (
canesten) in the area on the lesion and 2 cm around it twice a day for some days
4. If the lesion is improving in 4-5 days than continue the antifungal for another 5 days
5. If the lesion is not improving than try an antibacterial cream with fuscidic acid or mupirocine
6. If no improvement with this treatment please see your doctor.
Hope this answers to your question and is helpful to you
Best regards,