My daughter has the same problem. She was putting talc under her arms to stop the sweating and it got worse. Now she uses antibacterial soap twice a day. Try using hair removal cream or waxing instead of using a razor. Shaving leaves the hairs sharp at the ends, whereas waxing takes them out at the root and they grow back with a fine soft tip, less likely to grow back on itself and infect the follicle, causing the spot or lump. Also you could use t tree wipes to keep them clean and use plenty of moisturiser on them.
hope this helps
Lisa x