My Last Period Was On November 9th, My Cycles Are

My last period was on November 9th, my cycles are usually 31 days and I am currently 6 late. My husband and I are trying to conceive. The last few days i have just started noticing my breasts are becoming sore, increased cervical mucus and my lower abdomen feels full and slightly achey but no cramping. I have taken several HPT but all come back negative. Im just wondering what could be causing this?