If you see that your order is in transit or delivered at a date earlier than when you placed it, it is highly possible that USPS is reusing the tracking number you have. Rest assured that once USPS scans the barcode on your order's shipping label, there will be a new tracking update.
USPS tracking numbers consist of 22 digits, so that limits the number of possible combinations. And coupled with the high volume of packages USPS processes on a daily basis, old tracking numbers are reused regularly.
If you don't see any updates after 7 business days (excluding weekends) from receiving your tracking number, please contact them. They will look into the matter and take care of it for you.