Welcome to HCM
We understand your concerns
I went through your descriptions. I think that you are over reading into simple happenings. You can ask anyone about this happening and they say it is quite normal. You can check your underwear everyday and you should find the same stains often.
Because these are normal happenings. The waste is being decomposed in the anus and the decomposition produces biogas as well as some fuel, it is the process. Please don't give much weight to these until you have loose motion
for considerable time, cramps etc.
Hope this clears your doubt. Available for further clarification.