Depends, is there anything aside from sick leave you have done wrong? I assume you are UK, if so then company's need to have three steps before firing, a disciplinary, a written letter of a disciplinary, then a meeting which can result in your last warning or a sacking, failure to do these can result in you taking them for unfair dismissal an can go to the courts
The only way you could be straight up sacked is if you have done something massively wrong, i.e shown up drunk, been on drugs, stole, got into bother with the police that will go to court etc
If you are in a temporary contract then they can just not offer you another one, but if you have signed a permanent one then they can't just let you go without warning as you will have past the trial stage since that's usually the first few weeks you start
Could it be they maybe want someone to work alongside you? Do you have a backlog that maybe they are wanting a second hire to help clear it?