I think your cat need medical attention if this problem still persists. Serous discharge could be due to rhinitis, some irritants or allergens in the environment, some foreign body in the nasal cavity, nasopharngeal polyps or mass. Some other systemic problems like respiratory infections needs to be ruled out. I think you should closely monitor her and if problems persists or aggravate, immediately consult veterinary physician. Eating grass and previous medical history also favors the visit to vet. Do let me know what ever comes out after your visit.