What's the difference between : У меня́ бы́ло немно́го рабо́ты and У меня́ не́ было мно́го рабо́ты. ?
The difference is the same as in other languages using different types of negation. Negation usually emphasizes the following word. When the verb is positive
У меня было немного работы(I had little work )
the focus is on the amount of work, that you still worked though not much.
When the verb is negative
У меня не было много работы(I didn't have much work, I didn't work much),
it means you weren't busy much. The focus is on the action.