If it doesn't respond to a combination of ice, rest, and NSAID's, it probably isn't tendonitis.
Try doing high intensity lifts if you can and see how you feel. For instance, a few reps of moderate weight squats and deadlifts (if you can do proper form), standing or seated calf raises, lunges, etc. You may also want to try doing a wall sit for a few minutes. Try those things and see if you feel better immediately after. I have found these to work sometimes and think that sometimes the pain / stiffness could be due to a muscle not activating adequately, and by doing some moderate weight activity it triggers it to recruit properly. That is just my thought and I am not sure if it is correct or not, but it does seem to help.
Also, you may want to try the foam roller and see if you can find any tight spots that may cause tightness which ultimately disrupts the normal joint alignment somehow. Likewise, try the golf ball massage on the bottom of your foot to see if there are any tight spots in the musculature down there that you can self release.