hey lynney lou back from work now,
I had exact same problem as you go back to your doctor don't give up took me months before they would listen, I had no periods for months and then I would come on my period for months at a time 4 months heavy bleeding was the worst it got, at the time I was 17 im 20 now but I got palmed off everytime with the your only young they will settle down in time try the pill for 6 months to regualte them, I tried 3 different types over 18 months and nothing worked. I was feeling so weak and tired and just drained I would cry for no reason it was horrible. I also noticed that I had really small hairs on my tummy.
Went back just after my 18th birthday and they finally agreed to do a blood test I had anaemia, i then insisted on seeing a gyno, I had the usual examination and then got sent for an ultrasound scan. Scan came back clear, I then got asked to go for another one this one showed that I had tiny cysts on my ovaries.
Its really frustrating I know how you feel, I haven't had a period now for nearly 8 months and don't know when I will have another one. I have been given iron tablets and told to eat a healthy diet with plenty of iron and hormone tablets and thats about all they can do said they will help when I come to have children if I have difficulites.
My advice would be to go back to your G.P and ask for blood tests and if possible for an ultrasound scan.
Hope I have helped you if theres anything else just ask.
Clare XX