Period or Miscarriage?

We've been ttc for only one cycle now, I was meant to have my period on the 5th of feb. It didn't show, I took a pregnancy test and it said negative. Two days ago (15th of feb) I started getting really sharp pains on and off in my lower abdomen. Then this morning (17th of feb) I started bleeding, I put a tampon in and I put a pad on as well , I soaked through both within two hours. It is a lot heavier than my normal period, and also the blood is a really bright vibrant red with a few little chunks in it, normally it is a more a brownish red. Does it just sound like a different kind of period or could it be a miscarriage?