Breast mass and pain

Hi all, this is my first post so I hope I cover everything :-)

About 3 months ago I discovered a hard, painless mass in my breast (upper outer quadrant) and decided to wait a month to make sure it wasn't just due to my cycle.. It was still there a month later so I decided to go to a doctor. I was referred to get an ultrasound and that came back as a build up of tissue, thus I stopped worrying. Fast forward to 2 weeks later, I started having tenderness in that breast and it had swollen to a whole cup size larger than my other breast. Since then the pain has gotten so bad I can't sleep on that side, the mass (or hard tissue?) has grown so it covers the whole outer side of my breast, it has grown so much so fast that I have stretch marks or change of skin texture I experience aching arms, fatigued to the point where I'm falling asleep by 5pm, and my neck lymphnodes are swollen.

I've been back to the doctors and the afterhours clinic but they just tell me there's nothing they can do as I've only recently had an ultrasound and it came back clear.

My other breast is becoming slightly tender which makes me think that it's fibrocystic but wouldnt that show up on the ultrasound? I have reasonably lumpy breasts in the first place but the mass is a lot harder and immovable than the rest of the tissue.

I just want to know if anyone else has experienced this and what the outcome was... I feel like I'm being silly worrying about this when the ultrasound came back clear but I physically just feel unwell and I want to find out what's going on.

Any advice or personal experiences would be much appreciated! :-)