It depends. You my have a chemical imbalance, or you may simply be down. Talk to your doctor about both possibilities.
There are several types of depression and different ways it starts. If you are going through a tough time in life it can cause depression, like loss of a job, someone close to you passed away, etc..Have you noticed any changes lately before the depression hit. Talk to you doctor about depression and what you are feeling. I hope you get the help you need. God bless!