Peter, whatever is going on, the variation between measurements (due either to your cells, or to the assay) makes it difficult to pin down an actual number at the moment. However if it continues to be followed over time you will see if there is a consistent trend.
Apparently in some cases this happens and the PSA then stabilises at a "good" value below 0.2, possibly because it is due to some non-cancerous cells which were left behind during surgery but have only recently started functioning again. Alternatively the data so far gives strong hope that even if the rise continues it is slow and thus gives excellent prospects for salvage RT if that becomes necessary.
Good luck, there is nothing you need be doing about it now except continuing to get tested at intervals.