I don't know how you could have replaced the USB port (do you mean the cable? The port is the part inside the phone), but the problem is most likely either dirt in the USB port or the center part of the port is bent far enough that one of the contacts is touching the outer shell.
Turn the phone off and remove the battery. Use a strong light, a good magnifying glass (preferably on a stand) and inspect the port. If the center part is bent, CAREFULLY move it back to the center with a needle or pin. If there's some dirt in there, CAREFULLY pick out each little piece with a needle or pin, being careful to not bend the center piece.