Panic attack out of nowhere

I live with my parents, and I don’t work. Living a very limited lifestyle, I had gotten to where I feel okay most of the time, as long as I don’t try to do too much. Then today, I had a fairly strong panic attack for the first time in quite a long time. I don’t understand why. The only thing that has changed is my sister moved out of the house recently to go to college. Now I’m scared it’s going to happen again. I felt panicky gloom along with a racing heart. Life sucks.