Welcome! I am sorry you are feeling this way.
A few things come to mind while reading your post. First, you say you have been feeling this way for a few months. What has possibly changed in your life over the past few months or right around the time you noticed these new low feelings? Changes in relationships, jobs, health issues? Given this is recent, it could be that something, someone or an event could have been the catalyst.
The other area I think is important to address is the idea that you are not sure what is making you feel this way. Many times the complexity of our thoughts and feelings about the world, ourselves and the people in our lives are not always on a conscious level so we think we have no reason to feel down/bad. Talking to a professional will help you begin to discover these deeper levels in order to understand what is going on for you as well as help you heal.
Given the symptoms you are currently experiencing I recommend considering seeing a professional therapist. You can search online for one in your area. A few good sites are Psychology Today, networktherapy.org and goodtherapist.org.
I hope this helps,