Thanks for coming to HCM.
-Did you notice any change in size -- increase and/or decrease in size during last 4 months?
-Is the lump painful?
-Is it freely movable?
What was the day of your menstrual cycle when ultrasound
was done? Lymph
nodes are present in armpit and get swollen due to multiple reasons. But if the ultrasound couldn't find anything, it should not be a serious concern as it's with good reliability. One possibility is breast tissue extension to armpit, which can change in size/swelling during different parts of menstrual cycle due to hormonal
changes. Small amount of breast tissue in armpit is normal.
Your doctor is doing blood work to rule out any possible infection which might make lymph node swelling.
Wish you good health!