Agree with Margo, she needs to be seen by a vet as soon as possible. I might well take her back to the first vet, because it does not sound to me like he did not think anything was wrong. He may not have been able to come to a specific diagnosis for a number of reasons. Perhaps X-rays did not (or would not have, if they were not done) shown a wad of paper in the intestines. But he did prescribe medication and special food, so he thought something was wrong. Perhaps he just needed more information, or he needed to be able to do more tests, in order to arrive at an answer. Sometimes exploratory surgery is necessary in a situation like this, and perhpaps either he or you did not think the situation was bad enough on the 27th to justify that. It might be bad enough now. If you desire a second opinion, then do try another vet, as Margo49 suggests. If there is an emergency after-hours vet clinic in your area, you might want to go tonight. If the problem is an intestinal blockage, every hour could be critical. Good luck.
Could be a blockage. I would be taking her to another Vet ASAP