What to do if I have irritating pain in my hindbrain sometimes. Is it likely I am developing brain tumor?

Pain. Brain tumors usually do not cause pain. It is more likely that you have an occipital neuralgia, a pinched nerve outside the skull at base of the skull. Speak to your doctor about a referral to a pain program where this can be treated.
Not sensible. The brain is unable to feel pain from abnormalities within it; it is our mechanism for perceiving pain and interpreting it from stimuli elsewhere in the body. So you cannot be feeling pain in your hindbrain. Perhaps you have pain in the back of your head or in your neck, and that is what you are actually reporting. Brain tumors can cause such pain, but so can a lot of other problems. See a doc.