Have you elected to not pay CPP on the Self Employment income? Here are the instructions on how to make that election https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/cpp/you-stop-contributing-cp...
As a CPP working beneficiary, you have to contribute to the CPP, unless you are in one of the following situations:
• You are an employee working in Quebec.
• Your employment income is not subject to the CPP.
• You are self-employed and a resident of Quebec.
• You are at least 65 years of age, but under 70, and have elected to stop paying CPP contributions.
It sounds like the issue is the three months of self employment income would have been subject to CPP if the above election was not previously made.