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.
