What should I do about feeling something in my anus during ablution or prayer, without being certain if it was gas or not? And does my saying "May God reward you immensely" to my friend whose car broke down constitute mocking religion?

It is clear that the questioner is afflicted with waswasah (whispers from Shaytan/obsessive doubts), and he must abandon it and pay no attention to it. He should not give any importance to what he feels in his anus unless he is absolutely certain that his ablution has been invalidated. What he said to his friend is not mockery of the religion.