Sorry you find yourself in this situation.
So I guess what happened is that some mets remained after the prostatectomy, which caused the raise in PSA.
Do you know if a scan was done to find where they were before the salvage radiotherapy? Often, they will assume they're in the prostate bed, and this is the area which receives the radiotherapy. Sometimes pelvic lymph nodes and seminal vesicles will be included - it depends what the original diagnosis was, and the histology on the removed prostate and any lymph nodes.
Note that you don't yet know if you have rising PSA - one PSA reading isn't enough to say, and I hope the next one shows you don't.
If the next PSA reading shows you do have rising PSA after the salvage radiotherapy, it would suggest the remaining cancer cells might be somewhere else - somewhere not covered by the salvage radiotherapy. I would ask for a scan to find them, although while PSA is still that low, finding them on a scan is often not successful. Some types of scan should be done before starting hormone therapy if possible. If the cells are in an area which can be treated with more radiotherapy without exposing same area as before (which likely already had max allowable dose), then further targeted radiotherapy treatment might be possible.
Anyway, wishing you the best for the next readings.