Hi Tamara and warm welcome to our forums
Thank you for sharing your story. It is good you've found your way here. I'm not a health professional so really can't give you a diagnosis. Talking to parents and going to a doctor is really the first step for someone to take. No pressure, only if you want to. It will help you to get a better understanding of what's happening.
If you really don't want to talk with your parents or a doctor, have a think about contacting KidsHelpline on 1800 551 800 or chat on line at www.kidshelpline.com.au/get-help/webchat-counselling
While you say, you have no reason to feel this way, often depression is like that. There is no reason, or one can't get to the bottom of why one feels that way. There is no shame or need to feel scared in feeling that way. It shows you have a caring heart. You're not alone in how you feel. Many here on the forums often talk about feeling sad for no good reason. Feel free to browse our discussions and join in if you want to. Again, no pressure.
Keep reaching out if and when you want to.