Chances are that the HRT triggered the bleeding and it's nothing serious. But all post-menopausal bleeding should be investigated to be sure all is ok. Interestingly, an atrophied lining can cause bleeding due to instability yet the risk of cancer is extremely low when the lining is thin. Of course, a thickened lining can cause bleeding and so can fibroids and polyps (which can sometimes make the lining appear thicker than it is as it can be hard to differentiate them from the lining itself). I hope this helps.