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Hello, I'm Robin. Welcome to JustAnswer. I'm reviewing your question now and typing up my reply. I'll post that in just a few moments.
When a person is collecting benefits based on earned income then investment income would not reduce the benefits. Pension payments, annuities, and the interest or dividends from your savings and investments are not earnings for Social Security purposes.
Factors that may, or may not, affect your retirement benefits include getting benefits while still working and not being at full retirement age. This does not appear to be relevant for your mother's situation.
You are correct, medicare premiums went up and that is most likely what has reduced her monthly amount unfortunately.
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