The phrase 'I was singing' is a correct and usable phrase in written English. You can use it whenever you want to say that you were in the middle of singing a song or melody at some point in the past. For example: Last night, I was singing at the top of my lungs when the power suddenly went out.
"I was singing, man".
"I was singing, telling jokes.
And I was singing.
I was singing in the rain.
If I was singing, then I wasn't even there.
I was singing when they gave me the option".
"I was singing 10 days later," she said.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK