Poor little Sushi. I'm so sorry for your loss.
I think it was lice. A few years I got a rescue boy and the same thing happened to me. I was holding him against my black sweater and after I put him down I realized I was covered in little white buggys. Even after taking a shower I felt icky for days.
Here is some info (from the PetEducation website):
Lice
Cause and transmission: An infestation with lice is called "pediculosis." Lice affecting rats are Polyplax spinulosa, and those affecting mice are P. serrata. Lice are transmitted by direct contact or infested bedding. Lice are insects that can be seen with the naked eye. They are flattened and possess no wings. The eggs may be seen attached to the hairs. Lice are very host-specific and do not tend to leave their preferred animal: the lice of mice will generally not affect rats, and the lice of rats do not affect mice. Neither can be transmitted to people.
Signs: Polyplax lice suck blood, so in addition to pruritus, hair thinning, and self-trauma, affected mice or rats may develop anemia and become weak and lethargic. Lice can transmit blood diseases: eperythrozoonosis in mice, and haemobartonellosis in rats. Rickettsia typhi, the cause of typhoid fever, can be transmitted by lice. These blood parasites can be more deadly to your pet than the lice.