I'm a 2nd-year PhD student who's just come back from an academic conference. I'm feeling really sad now.
I had been doing my PhD project alone since the start of my PhD program, and I made quite a lot of progress. But three months ago, my supervisor said that he would like a senior PhD student to work with me. I said 'fine, I'm gonna have a new perspective.' So a final year PhD student, let me call him 'Sam', joined this project. Sam has his own PhD project, so he doesn't have too much time for this one, and he has been making far less progress than I have.
A month ago, my supervisor registered both of us for a conference. A few days after the registration, he told me that he asked Sam to make a poster about our project and Sam had already started to make it. He said Sam might need some of my data to make this poster. I was shocked. I didn't understand why my supervisor asked Sam, not me, to make this poster - this poster is about my PhD project! I'm the main player in this project, and he just helped a bit, so what on earth was my supervisor thinking about? Also, this was the first time for me to attend a conference, and I'd never made a poster before (Sam had made quite a few before), so I needed the opportunity to learn and practice to make an academic poster. By no means should my supervisor have asked Sam rather than me to make this poster, right?
I was quite unhappy at that moment, but I'd been trying not to think about this issue any more. However, the poster session at the conference yesterday was really annoying! Many other group members (we're a huge group) also attended this conference, and one of them saw our poster, whose first author is Sam, and said to me surprisedly, 'I thought you were doing this by yourself! Was Sam guiding you through these experiments?' 'No,' I said, 'I learned all this stuff by myself. Sam has just joined. He hasn't done much to this project.' But she looked unbelieving. During the poster session, our group members and other conference attendees asked Sam loads of questions but almost ignored me. And Sam behaved like he was the main contributer to the project. Now the conference is finished, and I'm sure people have believed Sam has been leading this project, not me. I feel distraught. I don't understand why my supervisor made Sam the maker of the poster. Does he prefer Sam to me? Does he trust Sam more than trust me? I'm too scared to ask my supervisor these questions because I fear he says 'Yes'.
So what do you guys think? What's the possible reason that my supervisor chose Sam over me?
By the way, a few days before the conference, my supervisor said he wanted us to write a paper about this project. Sam looked excited and quickly proposed an outline for the paper. So I wonder if my supervisor's gonna make Sam the first author of the paper....I feel so scared now.

You should ask your supervisor directly about this.

Talk with your supervisor and just be polite and honest about your concerns.

On a side note, it sounds like Sam is taking initiative and you aren't. Nobody gives you anything if you don't ask. You have the data so why don't you write a draft paper? You don't need permission to ask to do something and it is easier to ask for forgiveness than permission.

You should have asked your Supervisor from Day 1, what the implications of having Sam were. Always always protect your data from third parties, by asking in writing, detailed explanations for anything. But it's not too late. Ask Sam what kind of working does he foresee so you read the mind of your Supervisor. I would still schedule an appointment with the said Supervisor and discuss this!