Welcome to health care magic.
1.If you are already taking
Sulfamethoxazole drug for treatment - thats fine no issues, but it will be less effective for this kind of infections.
2.
Abscess means well defined infective collection with pain and tenderness and fever like symptoms.
3.The lump in the underarm / axillary region most probably an hair
follicle infection.
4.However as you are posted for
ultrasound - it will assess the nature of the lesion and its source and extensions, after that a small
incision and drainage will be done followed by a course of antibiotics will be given.
5.Mean while as a remedy do not press or interact the lesion as you will only complicate the lump, keep area hygienic and soft clothing for better healing, avoid shaving until it heals use trimmer instead - asme thing applies after the procedure. Good luck
Hope i have answered your query.
Any thing to ask do not hesitate. Thank you.