Cramps, period, and very heavy flow

I usually start my period at the end of each month, but this month I started on the 17th. It wasn't how my normal period starts. It was very light and somewhat brown like spotting. Then a couple days later I began to get a heavy flow with tons of cramps with usually is what happens when I do start but this has been going on for five to six days now (the heavy flow and cramping). I have to constantly change pads in an hour because I'm leaking through everything. Just recently today I've had a huge blood clot come out. Has any one else had this go on? What's happening and should I be concerned?