Gyno said yeast infection. gone for a day came back reoccurring discharge and sever itching. is this an std?

No: Yeast causes severe itching. The fact that it left for a day (with treatment i assume) is a good sign that it was yeast. Sometimes you have to use the treatment for twice as long as recommended, or repeat it right away. I would advise using treatment for longer. And no, it is not and std although sex can cause the overgrowth of your own yeast, and give you these symptoms.