This is not a guarantee that you have depression, but it is probably a good idea to consider reaching out to a mental health counselor.
While feeling sad is a normal part of life, these are typically triggered by a recent event (a break up, death in the family, loss of a job, etc). When these feelings continue to persist long after, or independent of, a triggering event then this is a sign of depression.
Other major signs include physical symptoms and little interest in pleasure in activities you used to enjoy. These symptoms do not manifest in bouts of sadness.