Sounds like a problem with your USB connection.
Are you still using the same computer? Perhaps you are now using a computer with USB 1.1, which is much slower.
If you are still using the same computer, perhaps the USB drivers got messed up? Also, often you can't mix USB1.1 and USB2.0 devices on the same hub. If you do, all devices are throttled down to USB1.1 speeds.
Even if you are plugging your Nano into a dedicated port on your computer, sometimes all the USB ports are actually connected together inside the case, as a hub. So plugging a USB1.1 device into any port could slow down all your devices.