It doesn't help to panic, but the answer to your question is YES. There is a slight rejection chance even after receiving PER. PER is an acknowledgement that the documents you provided are according to CIC standards and that you met the 67 point threshold. Background verifications, medical tests, security clearance all happen at the VO level and one can get rejected if any of these are unsatisfactory.
If you are confident that you provided 'real' documents and did not make stories up regarding your work history, and if you feel that you are in the clear with no security issues, then you don't have to worry about anything.
I received my PER last week and am not even sure what my final outcome will be. So chill, and make the best of what you have today. If you deserve it, there's no stopping you.
Good luck!