It would certainly be prudent to have the filly checked by a veterinarian to determine if she truly does currently have a hernia or if there is just edema or thickened membranes from where a hernia healed on its own. If there is a hernia present, there is always the possibility that some of the intestines could drop through the opening and get strangulated. Being in foal or performing strenuous activities could exacerbate the possibility of this happening.