Do you have pull up resistor to VCC on the 2 I2C pins? what is the voltage on the other side of the resistor?
The logic high level for I2C is set by the Vdd supply for
the pullup R's. If you are using internal pullups they get
powered by whatever the PSOC Vdd is. If you use external
pullups and terminate them to 3.3V, noise margin in.
this situation significantly reduced. The min "1" level is .7 x Vdd