Weight affecting my cycle?

So a few month ago I was ill and due to this I lost a lot of weight. I'm an 18 year old student at 5ft6cm that now weighs 7stone8lbs.. which is classified as underweight it says online. But I never weigh myself for that matter, I only weighed myself for the fact my period is nearly a week late.. could this be a reason why?
I've lost 11 pounds in two months- exactly to this day!