Unsure if pregnant or something else is the issue

Generally I have a 28 day cycle, give or take a day. My husband and I don't use any form of birth control. I was supposed to start my cycle on the 16th of December but have not. 
I've been experiencing symptoms such as dizziness, nausea, sore breasts and the occasional food craving. I've taken two hpt, one mid-day, which came out negative and again first thing in the morning about four days later, again with a negative. 
I'm not sure if I should still have a notion of possibly being pregnant or if there may be some othe issue. 
*there has been no added stress, no change in weight or diet*
Thanks for any help!