First, use the tube and valve stem to measure the distance along rim (each way), to determine probably fault area along the rim for closer inspection. Any signs of stretching or something on the tube?

Did you reuse the tube from the previous tire? What kind of tools did you use to remove and reinstall the tire? Screwdrivers? It is possible you pinched the rim during the installation. That usually creates a big hole, but possibly a small one too.