Your problem indeed is a typical problem. What I gather from your question is that you are getting a feeling of gas in the
rectum when penis entres your vagina. However, you do not pass the gas. When the penis is withdrawn, this feeling goes away. I shall answer presuming this is true. If my presumption is different, you may ask me a direct question on Premium questions, or directly.
Vagina and rectum are closely situated. There is only a cul-de-sac between them. Therefore anything in either of the organ is felt in the other one. So penis in the vagina will be felt in the rectum.
If rectum is sore, it will give sensation of fullness even though there is nothing in it and something from the vagina is pressing it. This sort of discomfort is usually felt in amoebic infestation. Your feeling of gas seems to be due to such rectal soreness.
Please get your stools checked up with
medical advice. I feel anti-amoebic treatment should give you relief.
You have not mentioned your age, how many children you have and whether your doctor anytime has mentioned to you that you have prolapse of the rectum through vagina (
rectocele). If so, you may require treatment for that.
Dear, some more details of your health history can give us many more clues to solve your problem.
However, I feel, currently I have given you some guideline. Please give me a feed back.