I am having a severe pain from that time.

I have my first child three years back. After that everything was absolutely normal. I had my second child in July this year. After that everything was just like before. I started my job after some time. My periods have been regular since then. Recently I literally got the worst period. I got my periods a week ago. I woke up this morning bleeding dark red heavily more than usual. I can feel like some clot of blood coming out. Anyone know what this could be? It makes me nervous. Just a couple days ago during sex it felt like my bf actually hit it and I was cramping but no bleeding. Now all of a sudden I'm bleeding heavy dark red. Any ideas on what's going on? I am really worried. I am having a severe pain from that time.