1.5 yr old female spayed cat, throwing up yellow& clear and foam?

I have a 1.5 yr old female spayed cat. A month or so ago we switched food -didnt see any problems. We went out of town for 3 days....had our neighbor check on her as normal. A week after our return she pee'd on clothes in a laundry basket. Then she started coughing and throwing up (she did pass a large hairball). We took her to the vet after a day of vomiting. He said she was fine and to call back in 2 days. She never ate and began vomiting bile and foam. We took her back - he said blood work was all fine, xrays showed a small soft mass in stomach which he thought was another hairball. She was given an antibotic just incase, and an IV. Two days later she still wasnt eating so he said he thought she would do better at home. Day one she took her hairball meds, drank and ate. Day two and three she ate very little. She has now stopped eating again... has thrown up 4-5 times - all bile, and foam. Her litter box is much less used. She is the only cat....inside.. no chemicals. Any USEFUL suggestions for when I call the vet back?