Do you have any pictures? Sounds like a hernia from the discription. If you feel it can you feel a hole in her abdomen where her "belly button" would be? Do you feel anything other than liquid in it, the dangerous part about hernias is the possibility of part of the bowel distending into the pouch on the outside. We've had one hernia over the years when a mare stood up too fast, caught it early and the filly wore a tennis ball taped to it and it closed on it's own. If it is a hernia at your filly's age & the size described it will need surgical repair.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.