Based on your report, really no way of telling of what caused this condition.
While the bowels are a gas containing space, their usual response to expanding gas is redistribution without signs or symptoms. On occasion this may result in abdominal fullness/pain, belching & flatulence, but bleeding is not expected in an otherwise healthy individual.
There is no apparent reason to suspect that it was related to barotrauma or other form of dysbarism.
It is good news that this event was several years ago, a single occurrence that rapidly resolved, involved no other signs or symptoms, and was worked up by 2 physicians without additional findings.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such. If this sign should recur, see a physician immediately.
Best regards.
DocVikingo