Hi. Im 16 years old and had some bleeding which i thought was my first period 6 months ago, i have had nothing since. Is this normal?

Also i have started feeling dizzy when I stand up to quickly and always feel fainty when i stand up for to long.

Im about 5ft 7 and 8st3. Little under the normal weight but i eat a good diet and alot of food.

I have also started to get bad stomache "cramps" and i dont know what the cause is. Should i be worried?