I had that happen in my 12th week of my 3rd pregnancy. I went to the er and they said it could be old blood! everything turned out fine, but i would still urge you to see your doctor! Good luck!
I haven't been pregnant, but my sister has been pregnant 4 times. One baby miscarried and she nearly bled to death before the ER would even check and see how much blood she was talking about.
I hope you're okay, but I think you need to call your your Ob-Gyn immediately.