Breast lump

Hi all. 
about  five days ago I noticed a lump in my upper right breast. It's not completely round but it's quite smooth and it's hard.  It feels like it's sliding under the skin but it's hard to tell because I can find one edge of it right near the outside of my breast but I haven't found a well defined other edge (if that makes any sense). I don't think it's particularly big. I'm not due my period, have never had anything like this before. 
I'm 33 and my mum died 4 years ago from secondary breast cancer. So as you can imagine I'm terrified. I'm going to the GP on Monday but the wait over the weekend is doing my head in. I'm assuming I'll be referred to a breast clinic, given my family history. I'm trying not to think the worst but I'm really struggling not to. I don't know if I'd be being naive to assume it was something else... I know only a doctor has the answers but I'm still googling everything willing for a harmless explanation x