Quick help needed once again...

I made the following pre-ferment last night:

50g mature 100% hydration sourdough starter
180g water
130g flour

This morning, after about 9 hours, it passed the float test, however I wasn't ready to make the dough yet.  3 hours later, so after 12 hours of fermentation, I was ready to make the dough, but now the pre-ferment seemed more liquid and thin.  I tried a float test and it sinks.  What can I do to save it in a shorter time?  I want to make the dough today and don't have time to wait another 10 hours.  Also, why would this have happened?

John