Couple thoughts

It’s already been touched on but you posted the same pic in a few different subs in a short time frame, that can come across as spammy. Lemmy overall isn’t as active as say reddit, so depending on how people browse, what they’re subscribed to, etc. that could mean you’re taking up a pretty big chunk of their main page which can be annoying when you’re seeing the same thing a half dozen times.

Some people are just weird about NSFW posts in general and downvote it when they see it.

I’m pretty sure some people also tend to downvote clothed/bathing suit/lingerie pics in NSFW communities because they think it should only be for nudity.

I’m no art critic, but in my opinion it’s just not a particularly good photo. Not that you yourself are unattractive but I just don’t think it’s a particularly flattering photo.

And on titsonastick, I’d argue that it’s not a particularly good fit for that community. Not that your pics don’t necessarily belong there in general but I don’t think that picture does a great job of showing off how skinny you are vs how big your boobs are.

And finally complaining about or questioning downvotes is a pretty good way to invite assholes to downvote you more

For the record, I haven’t down or upvoted any of your posts. It’s not necessarily my personal taste but I don’t find it egregious enough to warrant a downvote. People have different personal philosophies on how they vote, I tend to scroll past most posts and comments and only up if I find it to be particularly good or useful, I’ve enjoyed lots of posts that I didn’t bother to upvote, and only tend to downvote outright spam, bad faith arguments, and outright racism and stuff like that, but others will vote on pretty much everything in front of them.