It could be that what came out was a hunk of cervical mucus mixed up perhaps with normal vaginal lubrication. So long as it was clear and not discoloured or smelly - just exceptionally watery and moist then nothing to worry about - quite normal to have the occasional such episode, and having the cramps before it appeared to run out would add to that being the likely issue. the cramping will have dislodged it.
There is a less likely possibility that this was a cyst (not necessarily the same as the one you know about) which burst right at the end of the fallopian tube spilling its contents in to the tube and that flowed through the uterus and cervix and vagina to the outside world. It is not impossible but it is much less likely, than the fluid simply being mucus that was eased on its way by the cramps. an exploded cyst is excrutiating and i really think you would have noticed if one had burst suddenly rather than the slow leak of the one you know about.
some BCs work to thin the menstrual lining month by month at the same time thickening the cervical mucus production,so again being on BC would also account for this being a mucus issue rather than anything else.
I wouldn't worry about it. i can recall several episodes where clear fluid discharged from the vagina in a bit of a flood, rather than the usual discharge consistency.
and I wasn't on BC pills or had cysts that i knew of either. It just happened from time to time.
And beastly cysts do wreak havoc with period cycles anyway -so please don't worry about being early or late while there is a leaking cyst issue. There will be a lot of hormone disruption due to the cyst, and anything is possible with your period cycle inspite of BC pills.
i just spotted this pfli.org/faq_oc.html
the 1st few paragraphs are useful info too. it's the cervical mucus thing again.