Ok I'm not a dr, but some of the things people are saying is ridiculous! Anyhow, if you are under stress or having things happening in your life that are affecting you it could cause those symptoms. To me it sounded like panic and anxiety attacks. A lot of times when you start to feel dizzy you get nervous wondering why it's happening. This can cause the brain to start over reacting which can make you feel more dizzy, nauseous, chest pain, tingling, ears to ring or feel like you can hear, ect.... It's because you are so worried that it could be something serious that you cause yourself to panic. Laying down can def make it go away or slow down. If you find that the next time it happens that after you lay down it starts to go away after 5 min it must likely is a panic attack. It could be many other things though like low blood sugar, anemia or maybe you are a little underweight for your height and size. 100 pounds at 5'5 is nothing sweetie. Tell your parents what's going on and first see if it could possibly be panic and anxiety. If it is a good way to learn to deal with it is when it starts to happen tell yourself you are ok, you aren't going to die and it's just a panic attack, concentrate on your breathing and while you are doing this lay down if you can or at leAst sit. But do always get checked out to rule out anything serious or that you may need medication for.
This sounds like syncope of adolescence. You just need a tilt table test to get a diagnosis. This is pretty common. You need to understand it so you don't fall down and get hurt. That is the biggest problem with it - blacking out and hitting your head.
I'm no Dr., but in terms of probabilities, of which I am somewhat knowledgeable, the chances of this being a brain tumor at 15 are VERY, VERY, VERY remote. Regardless only a Dr. can diagnose the issue. The statistically most likely scenario based on those symptoms would probably being simple anxiety followed by low blood sugar but the later can be hard to diagnose. There is a test called a glucose intolerance test. It takes three hours with several different blood draws, one baseline and 2/3 more each hour after ingesting a sugary drink. These are only two of a material number of things that match your symptoms, so, once again only a Dr. can say.
that sounds like a brain tumor to me. im not positive but next time you go to the dr ask if he/she will check for a brain tumor.