Any tips on how to control anxiety?

I've been diagnosed with anxiety for almost 2 years now and went to therapy for almost a year. I've seen a lot of improvement on myself and have felt less anxious with time, but today, in the middle of school, I had a random panic/anxiety attack. Out of nowhere (and without any trigger) I felt dizzy and my eyes started tearing up. I had to leave the classroom and began to cry and couldn't seem to stop. After some while I composed myself and tried to carry on with my day to day activities but felt extremely exhausted. Any advice on how to control these random bursts of anxiety?