It could be stress. Or it could be indicating something more. I would suggest scheduling an appointment with your primary care doctor and discuss your symtoms with him/her and perhaps your doctor can then rule out other things that may be going on.
Teen aged years are not only stressful, but highly demanding, with new challenges academically, socially, emotionally and physically. You may be overloaded with all of these challenges and this might be contributing to forgetfulness.
Here in the US, they work with kids on organizational skills to help set up reminders for things and help kids prioritize busy schedules.
If you are truly concerned about memory problems and want to see a doctor about it, I would recommend that you start writing a list describing the kinds of things that you are forgetting and when they are happening. Without this, a doctor would probably dismiss vague "forgetfulness" as normal in most patients. If you have a guidance counselor or trusted teacher at school, you might want to share the list with them first; they might tell you these things are normal and suggest tips and tricks to cut down on the forgetfulness.