Ive had a hard, immobile lump in my upper outer armpit for a little over two months. lump is visible,feels attached to something, but disappears with skin movement & cant feel lump when arm is down. ultrasound found nothing. doctor said to ignore?

What are you truly concerned about after a negative ultrasound a doctors reassurance is not clear to me. It is not unusual to have reactive axillary lymph node that would come up if there was an infection and when done with its job it melts away. Check your breast if not done and examine for any lump, however the deep nodes may be difficult for some to locate the ultrasound would have shown it.