I had an urgent referral just before Christmas for a lump in my breast. The words "urgent referral" really make you absolutely crap yourself, don't they? But it's pretty standard. Just sounds terrifying.
It was a lump that didn't hurt, and didn't seem to move around, which Dr Google told me was not good. (Dr Google is a bastard. My doctor is much nicer, I shall go talk to her in future.) And I had a feeling it had been there for some time (breastfeeding, and it only became obvious when my breasts settled down a bit - before that they were too engorged for me to feel anything in there properly.)
Like a complete wally I just worried about it for several weeks (trying to convince myself it was my imagination), then finally realised I wasn't able to think about DS getting older w/o getting upset, and went to the GP. Saw GP on Friday who made urgent referral, got appt through on Tuesday, went to breast clinic on Thursday, and got the all clear. All that worry and I could have got it sorted in a week! A bloody week!
You said it helped to have an idea what the exam might be like so here's mine:
Went in and met consultant, who admired DS gurgling away in his car seat, then had a quick chat, he asked me a few questions about symptoms, history etc then did a breast exam. He then explained that they didn't find that mammograms were terribly effective on lactating women so we would take a look with the ultrasound. So out with the gel!
He had a look and then showed me the screen and explained that it was a milk cyst, which are apparently incredibly common in breastfeeding women but harmless. It just looked like a big dark bubble in my breast. He then used a very fine needle to draw all the fluid out (I could see the cyst shrinking on the ultrasound screen!) until there was nothing left. After that he said he'd send the fluid down to the lab for them to do some checks straightaway that we'd have results for today, and they'd do some more checks afterward.
He asked us to come back in an hour for the results, so went off and got a coffee (helpfully, navigating around the maze of a hospital to find the coffee shop used up some of the waiting time 
). Results were all clear. He said they'd do some further tests to be absolutely sure, and would send the results to my GP in a week, but there was absolutely no need to worry about these as everything looked good.
Good luck on Thursday catgirl, hope you have the all clear - it's by a long long way the most likely result, but always worth being checked all the same. I could kick myself for wasting that time worrying about it instead of going straight in and getting checked - well done you for getting it sorted quickly.