Girls: what bra size am I?Watch
Bravissimo have some really good guides on how a bra should fit when you wear it for the best support
The sizes you've been given at fittings will give you a very rough idea, so try on bras around that.
Also remember if you try on a bra and need a bigger backsize, the cup size should go down, if you need a smaller back size, the cup size should go down as the cup size is proportional to the back size (really hard to explain, but trust me, this is how it works)
Find a bra where the band roughly matches your under-bust measurements and do it up using the middle clasp (so it's not the tightest, nor the loosest) fitting around you. If that feels comfortable (not too tight, not too loose - you should only just be able to notice it's there) then that should be a good fit for you.
Make sure the straps are the correct length. The band should sit at the same level all the way around you - so many women have the straps too tight so at their back, the band rides up, and so the bra doesn't fit properly. Make sure the straps are loose enough to allow the band to sit straight round you, but not too loose that they fall off your shoulders: they should come straight up from the top of the cups to your shoulders. But make sure the straps aren't digging in to your shoulders either, in which case they're too short.
In terms of the cup size, that can be a bit of trial and error. Go for band measurement and strap-length first, then work out which cup size best fits you. If it's too tight...you'll notice because it will be uncomfortable and may make you look like you're in one of those period dramas like 'The Tudors' or something! Too loose, and it will gape around the edges (mainly near the top - if you look down at yourself and can see gaps where the cups aren't touching you, chances are it's too loose)
This should give you a good guide for what fits.
In terms of your bra size fluctuating throughout the month, the best idea would be to get a bra for when your size is 'normal' - when you find it's not feeling uncomfortable. When you notice it's feeling too tight, go shopping for another one you can wear around that time of the month, and size it in the same way as above. If, however, you only find the band feels too tight, you can get bra extenders (should be in the same area of the shop, usually on the end of an aisle) which hook in to the clasp and will give you an inch or so more band, without you having to fork-out for a whole new bra size. If the cups are the part that feel too tight but the band is fine, buy another size bra which has a larger cup size but the same band fitting.
Hopefully this helps, and sorry if I've made any mistakes as I'm doing this from memory of what my mum's always told me. But hopefully this should give you a good idea of the 'ground-rules' for how to size a properly-fitting bra.