Common? Well, no it is not “common”, but don’t use “common” as a measuring stick, because most adults, male and female alike, are not all that enlightened when it comes to making a woman cum.
Give these articles a whirl. They will explain what is happening when you have an anal orgasm. Yeah, they will rock your work. The source linked points out something I did not realize – that the anus is served by “the pudendal nerve”, which is the same nerve that serves the clitoris. Also, “the anus has two rings of muscle responsible for controlling digestion. The outer ring is in our conscious control while the inner ring is controlled by our autonomic nervous system, which is not in our control. As you move further inside the rectum any stimulation will be picked up by the same nerves that respond to stimulation of the inner vagina and cervix – the pelvic nerve. Stimulating this nerve brings deep and throbbing sensation, a feeling that many find extremely arousing.” So, if it feels like your G, it makes sense.
Don’t you love biology? Just like the Mayo Clinic’s motto states…”nothing like good ole’ butt sex.”