You have not mentioned about the disturbing thoughts in detail. These may be short lived & transient. But if these are present for a long time & affecting your day to day activities, then you may need to find the cause.They may be worrisome thoughts as seen in anxiety or pessimistic thoughts seen in depression or repetitive, intrusive, thoughts which you are not able to shrug off or you don't want to act upon but are worried that you may ,as in OCD.
Moods ups & downs are common in teenage, so are disturbing thoughts. But if it is affecting you greatly & making you withdrawn, need you need to visit a mental health professional.
Visit a counsellor who may assess you & refer you to a psychiatrist if needed.