attiny as i2c counter

Hi, all.
I am searching for a way, how to use ATtiny85/45 as a counter and transmit the data using i2c interface.
I found a piece of code for a water flow meter, able to measure the fall-down interupt on one pin and send the data using softwareserial.
But now I'd like to move to i2c and let the tiny work as i2c slave.
I am strugling with the i2c library for now - can't read the data using i2c get, but than I found out, that the usi i2c imolementation also depends on interrupt and I am affraid it will interfere in some way with the counter interrupt.
Did anyone tried to implement something similar?