Hello. I Have Pain In My Rectum, Which Came On

Hello. I have pain in my rectum, which came on following a journey in the back of a 4x4 vehicle whilst sitting on a hard seat. The pain is located just on one side of the rectum, and worsens during any tensing of the muscles in the area and also whilst walking. The pain does not noticeably worsen during excretion. The pain is sometimes quite severe and does not respond to haemorroid creams.

What might this be as a result of?