Hi Jill - I can sympathise with you here - the terrain between Sarria and Portomarin sure has its challenging spots.
I've had knee problems for years and need to be very careful. I've found out the hard way that the only way I can successfully avoid knee problems on camino is to walk by myself, or with another Camino snail - most people's pace is too fast for me; to take frequent breaks and to keep the distances around the 15km mark. This is really hard to do when you walk with a group where accommodation is pre-booked as yours was and the daily distances are probably greater, or when you walk with a friend who's a stronger walker, who's taller etc.
Two examples - I had a dream camino last June - Burgos-Rabanal - when I followed this formula - I walked at separate times with two dear, slow-walking friends and for a time by myself. I then had a not-such-a-dream camino last September-October on the Via Francigena when I walked with an Aussie friend, Sally, who was taller than me and who was a stronger walker. I tried to walk at my own pace, and Sally was very considerate in slowing her pace, and we did take regular breaks, but often we would find ourselves walking faster than I was comfortable with so I dropped back and Sally had to wait for me to catch up, which I felt bad about. In addition, the difficult terrain at times and the fact that the distances between towns were much greater generally, proved to be a problem - I longed for the Frances where the next town was never that far away. These combined factors proved too much for the weak knee and meant I had to drop out due to injury in Villafranca in Lunigiana after walking from Aosta.
Are you in training for your next camino at the moment? My advice to you, and you're probably doing this anyway, is to build up the distances and see how you go. If you are regularly walking 15kms, hopefully you should be OK. I wouldn't invest any money in knee braces etc. without knowing exactly what the problem is and for that you'll need the help of a medical professional as you know. Is there a physio locally that you could see? They will prescribe leg-strengthening exercises that will really benefit you.
One more thing ... did you use hiking poles to take the pressure off your knees?
I hope that your knee problem last year was a 'one-off' and that in your training and on future caminos you'll be able to take joy in every step.
As our wonderful t2andreo says, I hope this helps.
Cheers from Oz -
Jenny