Phobias are by their very nature irrational. As you are learning, they don’t make sense. Even after attempting to find the cause of your phobias, you were unable to. Logically, there is no rational reason to fear vomiting or diarrhea. It’s an unpleasant experience but there is no real danger.
Phobias are sometimes the result of an event that occurred in childhood. Sometimes they cannot be traced back to any specific event. Since you are having difficulty recalling an event connected to the phobias, try asking your family members. Maybe they will be able to recall these facts.
You can continue to search for a cause, but your time and energy would be better spent on treatment. Effective treatments exist for phobias. In fact, many of the treatments can permanently remove phobias. Two in particular, hypnosis and systematic desensitization therapy, have been shown to be effective. Neither treatment is lengthy and both have proven results. Cognitive behavioral therapy may also be effective.
You spoke to your therapist about the phobias and she had no response. You can try speaking with her again about your concerns. If she still does not respond, please consider seeing a new therapist that specializes in the treatment of phobias. As I mentioned before, they can be cured with the proper treatment. There is no reason to continue seeing a therapist who is unable to help you. As you noted in your letter, you can no longer tolerate (nor should you) the limited manner in which you have been living because of these problems. If you wish to search for a new therapist, please consult this directory. I wish you the best of luck.