As long as you were in a foreign country when you performed the services that gave rise to the income, it is foreign earned income. This would include severance pay.
The income is applied to the year in which you actually earned it, regardless of when you received it.
Therefore you would report the income on your 2018 return, and you would include the severance pay.
https://www.irs.gov/newsroom/foreign-earned-income-exclusion-2016
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
