Honestly, a hard and tender lower abdomen at one month post-C-section is quite common, the nerves that were cut during surgery take a long time to heal, and that can cause both pain and a firm, swollen feeling below the belly button. It could also be fluid retention, internal swelling still settling, or even something like gas and bloating slowing down the process, so there are a few things going on at once rather than just one cause. My own experience was that the area felt tight and uncomfortable for longer than I expected, and from what I've generally heard, most mums find it gradually softens as the weeks go by, though the timeline really does vary from person to person. That said, since it's specifically painful and not just uncomfortable, it's really worth getting your doctor or O&G specialist to take a proper look, just to rule out anything like fluid buildup or infection that might need attention.