I think it's just a case that all your stuff is drawing more then the 500ma current limit when using USB power.
Note that unlike most arduino boards there is no 500ma thermofuse to protect the PC's USB port from excessive current draw, so beware how much current your nano based project requires and keep it under the 500ma USB limit, or power the nano and/or extra stuff from an external voltage source.
My laptop has a current limit of 1A(1000ma). The volt meter reads 4.8v coming through the usb port with the nano plugged in. But after its gone through the circuits the 5v line ends up as 3.2v. The RX and TX lights flash even if the usb is unplugged. The program runs until the RX and TX lights start flashing and then the speaker crackles and the program resets. While the RX and TX lights are flashing, the 5v line goes to 2.3v and the 3.3v line goes to 2.4v. I have a 4.5v battery pack, where do i plug it in? 5v line, vin line?