There’s no problem with posting on different sites. It’s only a problem if you post someone else’s story from reddit here without their permission. Hope this clears things up, and I look forward to seeing what you post.
Any site that demands the exclusive right to host your story should be avoided. You are the copyright owner of your story and you should never give your permission to host it to anyone unless it is non-exclusive or you have made an agreement with them (usually involving financial compensation).