In basic terms, the vaginal canal runs parallel to the urethra (urine tube) that is above it, and to the anal canal that is below it. In other words, the three tubes run parallel and touch each other, with the vagina being in the middle.
If you are constipated or if you have not moved your bowels, it is quite possible that penile penetration and/or thrusting will be uncomfortable, or even painful (dyspareunia), because of the accumulated feces that is ‘hanging out’ in the anal canal. It would be more so if you tend to have lumpy, dry, bulky bowel movements vs. loose stools.
If this post describes you, take measures to promote loose stools and/or avoid engaging in intercourse if you have not yet moved your bowels.