sancelot:
powered with usb. There is an ams1117 chip that should make the conversion to 3.3v
No, there is not.
The Nano's 3.3volt comes from the USB<>Serial chip (CH340).
Don't know the curent capability, but it can deliver 30mA without collapsing (tried that).
Woudn't go any higher than that.
I suspect the DMM is giving the wrong readings (flat battery?).
Do what JohnRob suggested, and see if the 5volt supply is also off.
Leo..