Determining the Maximum Gate Voltage for an N-Channel MOSFET
Firstly you are using the source as an output and this is normally going to be about 1 to 1.5V below the gate (in order to turn the device on). That's OK because your gate will switch to 5V but some FETs may have a Vgs(threshold) that is more than 2 or 3 volts which means you might only see a logic level of about 2V on the PC0 input when USB power is on.

Secondly you need a resistor to pull the pin down to 0V when the USB supply disappears, otherwise the MCU pin will be floating around. I'd suggest a 10k resistor from source to 0V and try and find a FET with a Vgs(threshold) about 1V or less.

A better circuit is to have source connected to 0V with a 10k on drain to 3V3. This ensures that the fet (most of them) will more relaibly switch. The downside of this is that your interrupt input needs to be inverted - maybe this is done in code quite easily?