Hi Kate,
Sorry to hear about what you are going through!
I'm 14 and was 12 when my periods started. I can definitiely tell when it is 'That time of the month' as my pain will increase quite a lot and I never feel like doing anything - even getting out of bed is a huge challenge etc.
I already had RSD when my periods started but sometimes, they are either too early or too late, though they have never been as late as yours. I read somewhere that RSD can affect the menstrual cycle and my Doctor thinks that RSD could be hormonal so I guess that has something to do with it?
Have you seen your doctor to rule out any other possibilities other than RSD? I know we sometimes automatically assume that something is RSD and it isn't.
I hope you get some answers soon and you're in my thoughts! Please keep us posted!
Alison