On my pistol brass (.38 spcl and .357 mag) there is a small bulge at the bottom of the case just above the head after I re-size them a couple of times. After a few reloadings, it seems like the case may even be ready to separate in that area because of some "stretch marks." The bulge doesn't keep the cartridge from chambering, and when the cartridge is fired, the case appears completely straight. Am I doing something wrong, or is this normal?

Help!