Thanks all
Unfortunately I don't think it is a cyst. Or at least, it doesn't look anything like the ones on google images, (not to be recommended at mealtimes!

) It it completely covered with sound healthy skin, I did have a dog with a histiocytoma once before and that was much more like a warty cysty thing, it ulcerated slightly and it was like the entire surface was breaking down (it went away on its own BTW). This is a mobile swelling totally under the skin. I don't think it is/was an infection because there was no entry point, rather than it 'bursting' I suspect she caught it on something (possibly a vizslas tooth) and the substance the oozed out was part of the lump. It now has a small wound (not deep) that seems to be healing.
I will keep a close eye on it and consider if it need more vet treatment, my thinking is though (and my vet agreed at the time) that if I am not going to operate it is pointless, and for this particular girl a very stressful experience.
Thanks again for the input, I am slightly less panicky now she's stopped oozing!!