Well, first have a paw to hold - I've had lumps and thickened tissue and dips and been through all the scary moments, as so many of us have. And so far, its always been fine.
The odds of it being anything nasty are very low - you have a 90% chance of your lump being benign. And of the remaining few %, most are curable.
Also, big smooth moving lumps are usually 'breast mice', a type of cyst, both common and harmless 
It has to be checked just in case, but honestly I would bet on your odds - and I'm not a risk taker.
If you want a grounded, sensible crew to hold your paw while you wait, try the Tamoxifen thread - they are lovely, understand the panic, and have seen me through many a wobble!
Meantime, take a hard look at the facts, the odds, and be very very nice to yourself. Being anxious is unavoidable with boob related issues, but knowing you're statistically in a good place might help a bit 