What Could Be The Blood Soaked Paper Like On The Pad During Period?

I'm on the second day of my period and just changed my overnight pad and found a large chunk of what looks like blood soaked paper - not tissue paper, heavy fibrous writing paper. At first I thought maybe the pad had torn open - but no. I'm not on birth control, I don't use tampons, I'm not promiscuous - I've been married for 11 years, there's 0 chance of STD's - I had our fourth child 10 months ago and my period has been very heavy ever since.