Unusual, very intense period flow in 3 hours. What might have cause that?

I'm in my mid 30s and my period is always on time. The flow in the first day of my period is usually fair, two next days are heavy with cloths, and 2 last days are pretty faint. But this time aroundmy second day of my period was massive bleeding with bigger cloths. My towel was completely soaked and blood was all over my legs. It happend within 2-3 hours when I was in town running errands. Once I discovered my car seat stained in blood I had to go back quickly home and take a shower and wash my ruined jeans. I wasn't in pain or anything, and thankfully I could go to work (I'm on evening shift), and thank to God it didn't happend again later that day or next day. But, I wonder what might have caused such an unusual heavy bleeding? I haven't been eating anything different than usual. I would rather say I was eating less because of a lot of stress I have been dealing with for the past month.