In your last reply, you said you'd draw a full schematic diagram at the end of the day, but still haven't done so.
You said that you used a 220 ohm resistor for the LEDs "common" ground. Does this mean that you have 8 LEDs in parallel without individual resistors, and just one current-limiting resistor for them all?
A 220 ohm resistor should limit the total current to about 10mA, (and they'd barely be lighting up), if the LED Vf is about 3V, so if the LED current drain is affecting the Nano, something isn't right. Have you measured how much current the LEDs are drawing?
Did you change to digitalRead() for your PIR detector?
We're in the dark and can only guess what's going on from the information you've provided.