I began receiving social security in Apr 2018, and when I signed up the agent talked me into taking it retroactively, back to Dec 2017 when I first applied. So on my history online, there is a one-time payment for Dec 2017, and a one-time payment for Jan-March 2018, with the regular payments starting in April 2018.
Is this lump sum what they are referring to when they ask if you received a lump sum attributable to previous year, and if so, should I have received a separate statement from Social Security?
Yes, that is what is referred to as a lump-sum payment. You would not have received a separate statement from Social Security. If the payment was received in 2018, then it should have been reported on your 2018 SSA-1099. Since it was not, you should contact the Social Security Administration to see why it was not.
For now you should report the SSA-1099 just as it was reported to you. Do not add the lump sum payment on your own.
Yes, that is what is referred to as a lump-sum payment. You would not have received a separate statement from Social Security. If the payment was received in 2018, then it should have been reported on your 2018 SSA-1099. Since it was not, you should contact the Social Security Administration to see why it was not.
For now you should report the SSA-1099 just as it was reported to you. Do not add the lump sum payment on your own.