What Causes Constipation, Abdominal Cramping And Delayed Period?

I am a 22 year old female, I am 5ft5 and am 9 stone. I go to the gym and excercise regularly and go for a long walk every day so I would say I am in good health and I eat healthy lots of fruit and veg. I have not changed anything in my lifestyle but I have become constipated and have 'pellet' like poo. I have missed or have a very late period and I am having abdominal cramps. I have an IUD which I've had for six months but never had any problem with it before