Patient: "Hey Doc, it hurts when I do that."
Doctor: "Don't do that."
- Henny Youngman, 1946
Think of yourself as a science experiment. You have a dependent variable (the pain) and multiple independent variables (walking, swimming, stress) that you know of. In a controlled experiment, when all the independent variables are known, you remove them one by one to see the effect on the dependent variable (the pain) In your case, the easiest would be not swimming and walking on the same day and see what happens.
Another method would be to eliminate ALL the independent variables (stop everything) for a week or two, let your body achieve stasis and then add each one back into your routine to the point where the pain reappears, thereby defining the cause.
However, because you do not report the pain happening all the time, I would check two things:
1. Does the pain occur when ALL THREE factors occur simultaneously (walking, swimming on a stressful day)? If yes, you found the combination that causes your pain. Solution: take one of the elements out of the equation. If not, and this could be the case...
2, You have not identified all the independent variables in your pain equation. In this case, there must be MORE to the story beyond swimming, walking and stress. The way to figure this out is to journal. Write down everything you do, eat, drink, think etc. for a few weeks. Suddenly you will see a pattern emerge, one which identifies either a single cause or the unique combination that results in your pain. It's basic science 101: isolate the cause of the undesired result. How do you think we got to the moon? Same method.
Look forward to seeing you publish your results in the Lancet or JAMA.
PS: To make this work, you remove all emotion from the analysis. You're a beaker in a lab and you're about to turn up the bunsen burner... Do this right and you'll have your answer. No docs, no drugs...just the truth.