Tell the Players!
Your DM is making a mistake by hiding that you're possessed, and it's causing trouble for you, and, I suspect, causing the opposite effect of what they want.
The first part is obvious: the DM has put you in a position to do some cool roleplaying, but the rest of the party wasn't ready for this kind of thing to happen.
Campaigns where a character's actions might be influenced by things other than normal roleplaying need to establish this, in my experience. I DMed Out of the Abyss, which plays largely with the madness mechanic, and caused my players to develop all sorts of fun and weird quirks and flaws. But, I warned them that this might happen beforehand. The characters were unaware, but the players really enjoyed it.
The DM needs to stand up for you and explain what has happened, to preserve the peace, if nothing else. This will require everyone to properly commit to not metagaming off this information, but that should be standard at any table anyway.
The second part, though, is where I finally put my liberal arts degree to good use. The DM is going for surprise, but they're doing so at the expense of suspense. Alfred Hitchcock said it best in an interview with French film critic, François Truffaut:
There is a distinct difference between "suspense" and "surprise," and yet many pictures continually confuse the two. I'll explain what I mean.
We are now having a very innocent little chat. Let's suppose that there is a bomb underneath this table between us. Nothing happens, and then all of a sudden, "Boom!" There is an explosion. The public is surprised, but prior to this surprise, it has seen an absolutely ordinary scene, of no special consequence. Now, let us take a suspense situation. The bomb is underneath the table and the public knows it, probably because they have seen the anarchist place it there. The public is aware the bomb is going to explode at one o'clock and there is a clock in the decor. The public can see that it is a quarter to one. In these conditions, the same innocuous conversation becomes fascinating because the public is participating in the scene. The audience is longing to warn the characters on the screen: "You shouldn't be talking about such trivial matters. There is a bomb beneath you and it is about to explode!"
In the first case we have given the public fifteen seconds of surprise at the moment of the explosion. In the second we have provided them with fifteen minutes of suspense. The conclusion is that whenever possible the public must be informed. Except when the surprise is a twist, that is, when the unexpected ending is, in itself, the highlight of the story.
If the rest of the players know that there is a bomb under the table (a possessed party member), it gives everyone the chance to flex their roleplaying muscles, and can create some excellent suspense, instead of just a brief "gotcha!" moment, after however long the party spends being annoyed with you.
A Quick Caveat
All of this assumes that your table has a focus on collaborative storytelling rather than a curated, DM-told story. This will involve a lot of effort from everyone, but if everyone is willing to do it, I think this could be a really exciting twist.
What Can You Do?
Well, not a ton, if your DM isn't willing to listen. I'd send them the quote from Hitchcock, since he's got a bit more authority than me, a random poster with a Garfield icon. I'd emphasize the discord this is causing between the players, and that this tension may be hard to resolve. Send them a link to the problem-players tag on here, if they don't believe you; if there's no in-game explanation for this behavior, the number one piece of advice is to talk to the player, then have them leave. (Or advising the annoyed player to leave, if the DM won't deal with it.) You're not doing anything wrong, but the way it's perceived by your fellow players isn't going to end well. The party should feel betrayed by this, not the players.