Help, I haven't been diagnosed with anything nor am I self diagnosing, but I have a question which I cant seem to find the right answer for. 

I want to know if this is an underlying issue or a symptom of something, or is it just a personal thing I just do.

I will be doing whatever when I suddenly get the feeling of I'm not doing something, this leads me to go blank and stop whatever I was doing so I can go through everything in my head such as 'what I was doing/thinking' 'what I am doing/thinking' 'what I'm going to do' and 'what I want to do' Then I get a heavy feeling that I'm forgetting to do something or to think about something, despite there not being anything. After I get over this feeling or just try stop thinking about it and move on , I often later get that same feeling, it gets very frustrating. (There are days where I don't get this feeling at all)          
 (if this context is needed, I'm 16)