Yes my friend!!!

No, it doesn't mean that you are very sad, or even that you are sad. As Obakeng explained, we usually say this when we are dissatisfied or disappointed with something. For example, if someone's work does not meet the required standard, their boss might say 'I'm not very happy..'. Or if you want to complain about a product or service, you might begin by saying this.

When you say that it can mean that you are disappointed or unsatisfied depending on the context.

Not at all. In Chinese , it as well means you are in lost for something which perhaps you are not interested but insist due to many reasons ,such as responsibilty or expectation of parents and friends and so on . Simply it means you are free partly.