"We feel a little tired."
Translation:Nos sentimos un poco cansados.
Sentirse is reflexive. "Nos" is the object pronoun. "Nosotros" is the optional subject pronoun.
Well, my 501 Spanish Verbs book show sentir as meaning to feel sorry, regret or feel, whereas for sentirse it shows to feel (well, ill). It looks like if we are talking about how we personally feel, i.e., our own body, then the reflexive verb is used. That would call for nos being used.
No, the "un poco" part has nothing to do with it. Sometimes "sentirse" takes adverbs.
bien and mal [advs.] (rather than bueno and malo [adjs.]) are normally the words used to describe someone feeling good or bad.
The same thing happens with encontrarse or hallarse ("to find oneself [in a specific condition]"):
Me encuentro cansado (adjective)
Me encuentro bien (adverb)
This website might also help:
Correct form would have been nosotras NOS sentimos un poco cansadAs. If you're using the feminine nosotras instead of nosotros, you have to agree with the feminine cansadas, not cansados, and sentir is a reflexive verb that needs nos. As SaraGalesa said, if you don't include nos, the next best thing would have been estamos.
That's because of its method for picking the "closest" correct answer. Because you hadn't included the "nos", it chose its nearest answer which didn't include it (although it should have told you "cansadas"). There has been a request to show more intuitive "close answers", but nothing has come of it, as yet.