Periods

I'm freaking out... Since I first started getting periods they were always so heavy I'd need to wear super plus tampons & would last 6-8 days, but the past 2 periods my bleeding has been so light I've not even had to use a tampon, (sorry for being grotty) but I'd wipe after peeing and it would be like the blood from a spot you've burst & would only last a day/2. I've took pregnancy tests and I'm not pregnant. I've also got a coil... Does anyone know what the reason for this is?