Late period, negative test. Help!

I was suppose to get my period on the 13th of this month. There was nothing on that day and nothing on the 14th. Today is the 15th and I am just spotting. All three days I've been tired, lightheaded, and nauseous. I took a test this morning and it was negative. I feel very confused because my periods are always regular and on time. Could I be pregnant?