What Causes Pain And Pressure In Lower Abdomen And Rectum?

I have been getting stomach cramps and my bowel habits have been changing for about two weeks now. Also, I feel a sharp pain and pressure on my bowels when I apply pressure to my lower left abdominals. It is a sharp pain but only felt when pressure is applied while I am laying down. I am not sure what to make of this and am curious if it could indicate something g serious. No blood in stool, weight loss or nausea to report. I am an active 23 year old male.