Are the following sentences natural to a native ear?
1. Did you loose some weight? You look weak. ( I said that to a colleague)
2. Have you been sick, it seems you have lost a lot of weight. ( I said that to a colleague)
3. I need to get a haircut before I leave tonight.
Corrections as above.
There are many alternatives for "weak" you could use; I have suggested a few but many more spring to mind. "Weak", however, certainly doesn't work in this context. I'd also warn you about commenting on weight too much, many people will take offence even if none is intended.