What does it mean when submission system says "under review" after submitting minor revisions? Is it possible that after sending minor revisions my paper has gone for a fresh review again. I had received the following recommendation: accept subject to optional revision. The status has been showing " under review " for a month now. 

Is it still with the associate editor or with the old reviewers. I got two referred comments and one guest editor comments 
Sent my paper to ieee transaction
In most of the article submission systems the "Under Review" status means your manuscript is with reviewers. Most probably it must be sent to the old reviewers but there is also a possibility that editor might invite a new reviewer. It happened to me, one of my article which went through 2 revisions, 1 major and 1 minor took 1 year, and it was rejected because editor invited a 4th reviewer who raised few concerns. 
Best of luck for your article.