It is odds-on either for plagiarism or collusion.
In third year, and for a dissertation, being found guilty of either of those things can be very problematic and can result in withdrawal from the course.
This is very much a worst case scenario. However, consequences in third year are always worse than they'd be in the prior two years, even for a first offence.
Of course, it might be a misunderstanding and it transpire that there is nothing to worry about. Un til you are notified, you don't really know.