Lump in left armpit

[Concerned12348]

Hi, I'm not sure if I'am in the right place or not. I'am 26 years old I have had this lump in my left armpit for many years. I canno't see the lump (not like a pimple or ingrown hair), it is under the skin. You can only know there is a lump if you feel the area. The left armpit is also larger than the right, I'm guessing from the lump inside. (when you wear a bra..the fat that tends to stick out at the corner of your armpit..the left it sticks out a lot more). Like I said I have had this for years. Everytime I go to a doctor I'am told it is a swollen lymph node and its nothing serious. It does pain me sometimes. An uncomfortable feeling, like when my arm is down by my side, or sometimes laying on that side will bother it. And sometimes I feel nothing there at all, no pain nothing. But the lump is always there. I can't recall how many years exactly..but it has been a very long time. A doctor at a clinic once gave me antibiotics for it saying again that it was a swollen lymph node. I took all the medication, but still the lump was there. My new family doctor told me the same thing...swollen lymph node and believed that i'm usually getting the pain around the time of my period and hormonal changes are what is making it hurt. Recently it started bothering me and the lump inside feels slightly bigger. I went back to my family doctor and told her that I already had my period and was not due again for a few weeks. She is sending me for an ultrasound saying she thinks I may have fibrocystic disease. But when I do research on fibrocystic disease it talks about the breasts. (I do realize there is breast tissue in the armpit) But I have no pain or anything in my actually breasts. Only during the time of my period they get tender, but I believe this to be a common symptom of your period. I don't know if I should be more worried about this or not. I can kinda grab the area where the lump is, it is hard and not movable. I mean I guess this could be a swollen lymph node, but wouldn't that go away after a bit of time? Don't lymph nodes swell because of infection like the glands at the back of your throat when you get sick. But that goes away. This never goes away, is always there and has been for a long time.
My ultrasound is this coming thursday. When I called to make the appt. the girl said she was going to book me for a mammo after the ultra sound just incase they feel they need to do one.

Any advice would be great.
Thank you