Unfortunately, this isn't always the case. A doctor-patient relationship is like any other relationship -- sometimes the chemistry just isn't there. You have to make sure that you are doing your part to effectively communicating your needs to your doctor, though. You can do this by asking a lot of the right questions and by also by immediately expressing your concerns. Here is an example of effectively communicating your concerns to your doctor:
"Dr. Jones, I read that the symptoms I have are symptoms of cancer. I am worried that it is cancer and that my diagnosis is being delayed. Can you please tell me why you believe I do not have cancer? What makes you believe that I am only constipated?"
One mistake that many patients make is that they don't immediately discuss what they suspect is causing their symptoms -- they wait and see if the doctor does first. When symptoms are very general like bloating and cramping, it can it is usually because of something much less serious than cancer. For many reasons (personal health history, being low-risk, etc), cancer may not even be on your doctor's mind concerning your symptoms. However, if you immediate express to your doctor that you think you have cancer, she will address those concerns. (More Ways to Communicate With Your Doctor)
You may ask "What if she doesn't?". Ideally and most of the time, your doctor will respond with a thorough answer. If you do not understand any of the medical terms she uses, ask what they mean. Too many patients walk away not understanding their doctors because they aren't comfortable asking questions. If you do not agree with your doctor or still feel like she isn't meeting your needs, then perhaps it is time to get a second opinion or find a new doctor.