period?!? TMI!

I have a mirena IUD implant to try and limit my endometriosis until I can have surgery next year (I'm currently in year 12 and the doctor didn't want to interrupt my exams with a surgery). 
My period was a week late this month, and it originally started as a really dark brown/black colour, and has stayed that way for most of the period. 
It was bright red for a little while but only a day or so. 
I'm currently on day 7 of this cycle and The last 2 days it's been a yellow colour?? 
Is this normal or should I be concerned?