Random clot

On Monday I felt like I was spotting and went to the restroom to find my undies SOAKED with blood, filled up the bowl with blood and whipped and had a large blood clot (sorry TMI)... I wore a tampon and liner the rest of the day and did not experience one single drop of blood after that. Strange thing is I had my period about 2 weeks prior, I also am on my 4th week of metformin, have pcos and very irregular periods..any thought on what it could have been?