This is the easies pain to prevent. If you anus is stinging then there is not enough lube or you are using the wrong lube. What Makes a Good Anal Lube is an article dedicated to the subject.
Forcing either of the anal sphincters open before they are ready will cause this. It is exactly the same type of main that you experience when you “pull” any muscle. Once you strain an anal sphincter it is too late to back off and start again. The strained muscle becomes bruised, is very sensitive to touch and very painful if stretched. This is a show stopper. You can expect it to take a day or so for the muscle to recover.
Beyond the anal canal there are no pain receptors to tell you thing are going wrong inside. After you get past the sphincters and the Puborectalis, you may experience pain that is hard to pin down or describe. This is Referred Pain map. It shows where you may sense pain when internal organs are stressed.
Where you might feel pain when an internal organ is distressed.
Failing to navigate the Anorectal Angle pushes straight into the wall of the rectum above the anus. It may not solely be the rectum complaining. In the case of a male it could be the prostate and seminal ducts which are directly above the anus. For a woman it could be the cervix or uterus.
Body positioning and the angle of the penetrating object will dramatically ease the task of navigating the Anorectal Angle. If you lean back the kink becomes sharper. Crouching forward, literally try to put you head between your knees, will straighten this section out (See Getting through the Anorectal flexure above). Most of the time, internal distress from anal sex triggers a head ache, migraine, pain in the neck or back.
But we are not all the way in there yet. Next you have to worry about the intestinal folds. Use gentle gyrations as you insert further. Ensuring you don’t push the tip into, or behind, one of the rectal folds.
IF the Bottom experiences a sharp (internal) pain accompanying a “POP” sensation, you have forced through one of the rectals folds or the normal internal twists and turns. This usually result in the onset of nausea if you keep pushing things. To stop a savage POP and pain, the Bottom must do the pooing thing while the Top works on navigating the channel.
The basic navigation steps
The Top enters the anus aiming for the belly button. This is the natural line of the sphincters (the anal canal). Then you tilt the shaft gently toward the back to push the tip into the rectal cavity. You then follow the spine while not over stressing/stretching the Bottom/anus. It is important at this point that the bottom tries pooping things out. Stimulation of the rectal walls will trigger this automatic poo response. The Bottom just needs to go with the sensation. The rectum and anus will be pulled in line. As you go deeper, start tilting things towards you front again. Follow the curve of the lower spine and giving the Transvers folds time to get out of the way.