Hi Hayley,
Anxiety and depression certainly follow people with ADHD. (Its not knowing why you are feeling or doing what you are doing, that leads to the anxiety and depression). And ADHD is definitely passed down from one's parents. And, of course, the inability to concentrate is also a symptom. Having said that - anxiety and depression will also cause a lot of the symptoms you have mentioned. So the best thing to do is to find what is causing the anxiety and depression. You said you were recently diagnosed with that. Is the doctor also looking into the root causes? Hopefully you are seeing a psychologist or a psychiatrist and not just a general MD.
And, of course, all of this could be caused by the "options at school" coming up. If you are spending a lot of time studying, that will cause both the back and neck pains. Belly pains are usually due to anxiety. So I guess the question is - have you felt like this all year? or just recently? If its a more recent thing, then ya, its the stress of all the upcoming stuff. Of course, if you have ADHD, it's much harder to study and that leads to more stress.
Point being, ya gotta figure out what is driving the stress. Stress by itself (caused by events coming up) can be dealt with. Stress driven by ADHD can also be dealt with.
So what do you do. Well, obviously part of this depends on how long this has been going on. It might be a good idea to get more information on ADHD to see if any of the symptoms sound familiar. I've got tons of resources. While many are very detailed (your parents might be interested in those), this is pretty direct - check it out -
http://www.medhelp.org/posts/ADD---ADHD/What-signs-should-I-look-for/show/773154
If there is anymore questions you have, please feel free to post. Best wishes!