My accepted paper now appears as "under review" - could it still be rejected? I have a paper that was accepted at a conference and in publication of conference special issues of a journal. The acceptance email stated that we will be contacted by the journal editorial office for further actions regarding the publication. 

At the time of acceptance the status was “editorial assessment”
But this changed to “under review” again about a week after.
Now the status changed to “reviews received”. 
Why could this happen? It was already peer reviewed before. I am confused and anxious and it will get rejected. Any thoughts?
It seems to me that the conference committee accepted your paper to be presented.
The related proceeding is then  subjected to its own revision according to the journal standards.

This is what I get from your Q - in which is not that clear who has accepted what - and it is a common work-flow for many conferences when the proceedings are to appear in a journal, albeit in a special issue of it.

Alternatively, the editor at the journal could have decided to consult one more referee. This can happen, of course.
But in this situation you shouldn't have received any acceptance letter from the journal, yet, but only that coming from the conference organisers and regarding your actual presentation at the conference only. Hope it helps and good luck.