Initial steps or instructions to begin using Microsoft Q&A
I checked your public profile and it still shows 490 reputation points. That tells me this is the same profile that earned those points, so I would not assume you signed in with the wrong account.
The Activity feed on a profile only shows up to 30 interactions over the past 30 days. That note appears at the bottom of the feed. If your older posts are more than 30 days old, they will not appear there, even though your reputation stays.
I would look for your older content in the Q and A section of your profile instead of in Activity. That section lists your questions and answers. A post that was removed would also not appear in your feed.
If you last saw a post in the past few days and it is now missing from that 30 day feed, please add when you last saw it and whether it was a question or an answer. That detail will show whether this is the feed limit or a missing post.