Why do I get blisters one week after the laser tattoo removal session?

I started my half sleeve tattoo removal last year, using a q-switched laser. The first two sessions were great - almost no blisters, and great fading. I was super excited. Then the third time my arm just burst into horrible blisters and I had to be seen by the burns unit at hospital even though the settings were exactly the same. So I did two patch tests following that incident using lower intensities but blisters appeared again. Weirdly, this only happens 7 days after the treatment. Any ideas?