I usually feel sad and tired. could be a depression?

Fatigue and Sadness: How long have you felt this way? How old are you? Have you suffered any recent loss (death of friend/family, loss of a job, divorce, etc.)? It may be depression, or you may have a thyroid disorder, or other physical problem. Start with a physical exam with your MD, if everything is OK, consider talking with a psychologist about these feelings. Help is available, good luck!

Yes: You owe it to yourself and those who care about you to get this managed, have other causes ruled out (are you taking meds?) Treatment is likely to include both medications at low doses, challenges to do things that will help your brain re-program and make you happier, and tough-minded reality-focused study of your patterns of thinking

Check it out a bit: There are a number of reasons you could be experiencing fatigue and sadness. A great first step might be to see an General Practitioner or an Internist to get a full check up. Seeing a mental health professional is also something that you might want to look into.

Maybe: Depression can manifest in many ways. Here are the main symptoms: feeling bad about yourself, losing interest in hobbies/activities, fatigue, poor concentration, poor self-esteem, guilt, pessimism about the future, wanting to die or hurt yourself in some way, sleep problems, eating problems. However, feeling sad and tired can be caused by other conditions as well - consult your doctor.