You might want to think about the adjectives:
She was being silly last night.
She was silly last night.
Both sentences are possible and both describe her behaviour. I'd use the progressive in examples like this:
Don't take what she said seriously; she was just being silly.
Here, I don't think 'was silly' would fit in- it's describing the frivolity of her behaviour rather than judging her negatively.
- For Teachers