Can implantation bleeding occur 20 days into cycle?

No: No - bleeding 20 days into the cycle would not be implantation bleeding. While 20 days could be around the time that the fertilized egg implants into the inner lining of the uterus, my understanding of implantation bleeding is the type of pinkish spotting that women may experience after they anticipated to come on their next menses up to about 8 weeks by LMP.