No, what you described is not self-employment. If you rent your property below fair market value, the IRS considers that you do not rent your property to make a profit. It sounds like they need an explanation of the income. If you described it as house sitting, you implied that you were the one house sitting, which would be self-employment income. You claimed the income correctly.
Per the IRS, "If you do not rent your property to make a profit, you can deduct your rental expenses only up to the amount of your rental income. You cannot deduct a loss or carry forward to the next year any rental expenses that are more than your rental income for the year."
Per the IRS, you would report your not-for-profit rental income on Form 1040, Schedule 1, line 8, Other Income.
If you are filing Form 1040 and you itemize your deductions, you can include your mortgage interest and any qualified mortgage insurance premiums (if you use the property as your main home or second home), real estate taxes, and casualty losses.
For more information, please see this IRS link: https://www.irs.gov/publications/p527/ch04.html#en_US_2016_publink1000219164
