"She was losing weight."

Translation: Elle perdait du poids.

[Scott495798]

Why 'du' and not 'des'?

[EloddY2]

Because poids is singular here (le poids - btw the plural form is also poids).
In this context, you always use the singular form.

[Scott495798]

OK, another one of those things you just need to memorize, I guess. Thanks!

[Vic3141]

My brain is wearing out