You should be just fine! I live, work, and play in an area with whitewater rafting and I have never had a problem during a trip.
If your stitches are all healed up, then level III rapids should be okay to paddle in - they are perfect, not too big and not too small.
For extra reassurance, I would inform my guide and he or she may be able to place you on the raft in a position where you might not need to paddle as forcefully. Also, if the trip is long (where I live they are about 6 hours) I would take some Imodium or whatever works for you to slow the bowels down. Also, if they provide a lunch you may want to let them know if you have any dietary restrictions.
Hope you have a fun time! I wish I was on the river right now!

Of course you can! Get out and enjoy life...and let us know how your trip was.

Sue

Thank you for the advice and encouragement.
All went fine. We had a lot of fun. It was half a day trip starting at 2:30, so I had breakfast and then drank protein instead of lunch around 1pm. Used the bathroom before getting in water and I did just fine.
Thanks!