Why did my professor criticize my performance indirectly?
Your advisor said, in the context of a conversation on a new student picking up some of the workload in the lab,


  xslittlegrass is not productive as before


and you seem to have interpreted this as a personal criticism, i.e.,


  xslittlegrass is not working hard enough. I'm disappointed with xslittlegrass's performance


I question this interpretation. Your advisor is eager for the new student to arrive because she recognizes that there is a lot of work to be done on the current projects in the lab, and that your attention is on other things right now - hence your low productivity. That's not necessarily a bad thing. You simply can't "produce" a lot on "your" project right now.  You have been splitting your time and efforts between multiple things: your exam, the collaborative project, and "your" project.

Next year you'll be even less "productive" - you'll be writing your thesis, devoting time and effort to your job search, making sure ongoing projects transition smoothly when you graduate. That's a good thing :)

I also question your characterization of the situation as "under the obvious condition that I can hear it" - your advisor probably did not expect you to hear this conversation. As you said "usually I will close my office door when there is a meeting outside" - so usually you do not hear meetings going on outside. I don't think your advisor noticed your door was open, thought "xslittlegrass can hear this meeting," and decided to use it as an opportunity to give you some indirect criticism in middle of an unrelated conversation.

If you are really concerned about this comment, it's often helpful to have a "checkup" with your advisor. You can ask her directly: "Are you satisfied with my current work? Do you think I am allocating my time and efforts in a useful way?"