I discovered a large lump in my left breast last week.

I've just had a full abdominal hysterectomy and my ovaries removed so due to this, I've been showering as I'm not allowed a bath during my recovery. I found this lump after my first shower so I can't say if it was there before or not. 

I had just started using a Hrt patch and I'm hoping this lump is related to hormone changes.  It's only on one side. It doesn't hurt and it's large, like a plum and quite solid. It doesn't seem that mobile but it's hard to say. 

I'm at the breast clinic on Thursday and I'm wondering if anyone has any thoughts on what it could be.