Small cluster of blisters?

I have had several "attacks" with this. It starts with small blisters, maybe one big one or several small ones. The first "attack" was after I had my second baby(second c-section), it was on my knee...it burst a crystallized salt like substance. Later, I had this fluid to fill into my smallest toe on my right foot...PAIN! And several other single blisters on my arms, and left leg. Was tested for gout....which was neg. Tonight I have a recurrence of this small cluster of blisters again, on my wrist. What is this???