Hi it sounds like a cyst. If gets worst before period that's nature of cyst changing through the cycle.  Also cancer tumour is not painful and much more common for some reason in the left breast. They will be able to tell you on the ultrasound as cyst is liquid filled. Hope this helps

Cysts are usually soft and circular though right? This is rock hard and def doesn’t feel like a ball or oval shape 

Well I had a cyst in My right breast in 2017 and I had a cancer lump in left breast 2022. The cyst felt like rock hard to me , they say cysts are soft , mine wasn't. And painful throughout the cycle . If not cyst it can also be fibrodenoma . Like I said right breast less likely to be cancerous and the cancer tumour was never painful in my left breast. 

Also if movable gp said, that doesn't sound like cancer tumour. Those are not movable 

Thanks for responding! It’s a painless rock hard lump in my right breast. I think cancer lumps only become unmovable once it’s well developed in the later stages? 

No I had grade1 and it was not movable from the day I noticed.  If like you said ,worst before period doesn't sound like a cancer tumour. Period made zero difference to mine but cyst was definitely different around period

Thanks for info! Did your cyst feel like an obvious round or oval shape? I obv hope mine is only a cyst but really doesn’t feel like it 

It was like as if glass ball was placed under the breast tissue. That's what it looked like .also look into fibrodenoma quite common too. No point thinking the worst there are a number of things this can be. I know anxiety play tricks on you mind.  Reality is that it can be cyst ,fibrodenoma number of things. 

Then they say cancer lump is irregular edges mine was like a perfect oval stone but not movable and no changes around period and it was in left breast. Good thing about yours,  it's movable , changes before period . These are not the characteristics of cancer 

And its in the right breast