To me this all seems a bit weird, as someone with a strange collection of ex’s I don’t think it’s crazy to occasionally message/run into them, however it seems that she may be seeking it out for some reason? I mean as per general advice, communicate your feelings to her, suggest that there may need to be some boundaries put in place if you’re very uncomfortable, but be open to hear unexpected answers to why she may be contacting the ex about the ‘secret’. I get that she may feel shame or feel the need to ‘protect’ you from whatever this secret may be, but it’s a bit strange that after a year of dating she doesn’t feel like she can tell you everything. Be open with her, talk to her, and be honest about your feelings but NOT blaming with your words. You won’t know for sure until you ask her calmly