Usb cable weakens after just a week of use?

I didn't know its brand since I have thrown the box away. It has a thick wire and have read head and ends. It charges my 4k mah phone in just 4 hours which is impressive for me and very beneficial but after using it for like a week, it loses its power. Now my phone takes 9 hours to charge again. (Power on no screen) Is it a defective usb cable or just something went wrong? My phone is a Lenovo a5000