Got a letter from my boss

I started my first job 6 months ago, and I was on probation for 6 months. My boss handed me a letter 2 days ago just before my shift. On the way home I opened it, and the first thing it said was, " we have a number of concerns with your performance in your role" and they would like to dicuss it with me tomorrow. It was a very short letter. The outcomes could be me getting fired a warning or more time working there. I don’t think im getting fired because the same day I got the letter my boss asked me what holidays I want for the year and he seemed nice and relaxed when I talked to him. Am I in trouble? Any store managers can give some insights?