I’m still pretty new to Mastodon, so I don’t really know much about it yet. But, on top of hashtags that you follow, could it be posts that are shared (or boosted, as Mastodon seems to call it) by another user that you follow?

Aside from the hashtag hypothesis, there are multiple timelines on Mastodon: trending, local, and global for example. I’m assuming you didn’t suddenly accidentally start looking at a different timeline but still worth mentioning.

On the official app i can select either “local” or “following”, could you be on the wrong one?