What causes bleeding to have sudon changes

I was sapose to start my cycle on the 13 of dec. However I started on the 16 of dec. I have had pressure in my abdomen since 2 of dec. Im so confused im never late and I had sore brest and veins showing in them since 2 of dec. They are no longer sore. Im not bleeding as heavy as I normaly do I feel sick alot and get sick sometimes I havent taken a pg test cuz I started bleeding should I take one? Could I be pregnant? My cycle has been on time and the same since dec 2010 until now.