Dear user, These kind of new appearing lumps in armpits can come after infection in the arms, hands, chest, or breast, and they can be indicative of any systemic (generalized body) illness. And these usually resolve with the resolution of disease. As we don t know what did you actually suffered from during your stay in Thailand, but if it persisting, it needs to be checked. Usually that involves doing and ultrasound of your axilla (armpit), and taking a biopsy out of this lump to know the exact nature . You might need a few blood tests and x-rays / scans to locate the source of the disease. I would recommend get an appointment with your doctor and get the required tests done. Hope you got your query answered. please write back if you need to ask something else. Thanks