Dardanelle:
The situation you refer to is a troubling one, as I am certain you are well aware. I hope you have a copy of the pathology report from after surgery. This is packed with information which may point you to an answer. Without that information everything is conjecture. What did your surgeon say after his work? What does he say now?
It is possible that surgery does not remove all prostate tissue and some active, normal cells remain afterwards and this could be the cause of the psa. This is a benign condition. It is also possible that some residual cancer remains and will soon die as the effects of the surgery damage the blood supply for these cancerous cells and they too will soon diminish, along with a coincident drop in psa. This is a less likely combination of factors. The lab reading could be in error. Other men have posted experiences of such errors.
The pathology report and your surgeons comments or operating notes in your medical file should point to some explanation of this result. In any case, only more time can answer this question successfully. Prostate cancer exhibits psa which, over time, is a clear indication of the direction of the disease. As difficult as it is to allow time to pass, anxiety will change nothing. Subsequent psa tests will tell a fuller story, unless the path report and surgeon report makes clear where your situation lies. Your answers will come from your medical team. No one here, or elsewhere on the internet can successfully diagnose your situation.