Lump cannot be seen on Ultrasound

Hi, aroud 3 months ago I found an abcess on my armpit (luckily its gone now after I squeezed it, but I still can feel something underneath). And near to the abcess, I develop a new lump, hard lentil size lump that is very near to the skin, it can be felt easily. My whole armpit is swollen too, like it is slightly bigger than the other one. I went to see doctors for 4 times and none of them seems concern. So I went for an ultrasound today and the doctor found nothing on my armpits and both of my breast. I can feel the lump on my armpit, even I can see the bump on the skin. But I don't know why the ultrasound cannot pick it up. And for my swollen armpit, some said it is because of lymph node that swells deep inside. But still nothing found on ultrasound. It's been 3 months, and.. what do you think should I do? I don't think I can order for a biopsy since ultrasound see nothing.