What Causes Severe Pressure In The Rectal Area While Sitting?

Thank you. I am a 54 year old female, healthy, post menapausal. Several weeks ago (sudden onset), I began to experience pressure to my rectal area with sitting. My BMs changed in frequency and became somewhat constipated (a change) and then became soft (normal) but smaller in diameter than usual. Some abdominal distention noted. No abdominal pain. All lab work was normal. I went to the GYN, exam was normal. Had a colonoscopy, normal. Hemocult, normal. I continue to feel intermittent rectal pressure and BMs less frequent. Taking Metamucil daily to see if helps. Can you provide probably cause?