There are a couple of reasons that might have your Medical Status be changed from "Passed" to "We are reviewing your medical results":
1. If your medical exam expires in 1-2 months, CIC might request you to re-take your exam (if you're an Outland applicant), or they might extend it (if you're an Inland applicant);
2. If you added a new dependent in your application (i.e. a newborn);
3. The officer might be going through a final review of your application and thus changes all of your statuses on your CIC account again.
Either way, since your Medicals have been passed already, there is not much to worry about