Canuck4570,
if a #12 bow makes you feel more comfortable shooting, shoot it. I will differ than Jim on opinion. Shooting a light bow always helps help work on form and it always exposes release problems. If you can shoot a #30 bow well, you can shoot a #40(given that the weight is not too heavy) even better, id bet.
back to the OP, a #25 bow will work fine. Just try to find some arrows that are around 225 grains including tips and you will have a heck of a 3D bow. Some 3D courses can be pretty lengthy, up to 50 shots. You definitely need to have a weight that you can manage comfortably, and no matter what anyone says, it's your bow, and more importantly, YOUR ENJOYMENT!
You could have a number of problems going on. I have had bursitis pretty much my whole life. If I do too much of anything, I will get some pretty good joint pain. I am no doctor, and this may seem counter productive, but get you some type of thera-bands and do some work outs at home. If you strengthen the muscles around the joint, it will help.