Broken rear Strut spring

Hi there
I have a 2006 Prius with 75k miles on it . I installed a trailer hitch on it last week for doing some light weight hauling of things. I jacked the car up with a heavy duty jack that can lift one whole side of car up and put a jack stand on rear jacking notch I then jacked the other side car up and did same thing . I have done this many times before normally just for rotating tires . After installing hitch I lower the car slowly back down and than I sat down on the bumper to do the inside wiring. As I sat down I heard a big thunk. I got off the bumper and didn't notice to cause that noise so I proceeded to finish wiring and as I shut the hatch I heard another thunk. To make this story shorter , I looked at car and it looked like it was sitting lower in rear . Yesterday I jacked car back up to see what the trouble was and found the strut spring on the rear drivers side broken . Has any body every heard of this . Didn't hit any hard bumps to cause this and like I said earlier the car was lower down slowly and not just dropped down. Was there a recall for the struts or has anybody else heard of these springs breaking . Is there a better replacement for strut and spring assy. Local auto parts store don't even have a spring listed for replacement but they do carry the strut itself .