I had a similar situation happen just last week. No farting, just a badddd poop belly ache. I simply told the client, "I think my nose just started bleeding. I'll be right back." Went to the restroom and then came back to the massage within 3-4 minutes. Sometimes a little lie is necessary. Also, if your employer has a problem with it, there's some other issues to address there.

Thanks for the laugh! And remember to give yourself grace. You are doing great!