If your last period was on Dec 28 then your next should have come around 28th Jan. At times the cycles can delay be a week-ten days. Since you are having cramps you may start your cycle soon. At times cramping is present in early pregnancy too. Hence if your periods do not come for a week then take a pregnancy test. If it is negative then then go for a blood pregnancy test. A blood HCG estimation test would be more reliable. If the test is negative then you are not pregnant. If it is borderline, repeat after 48 hours to see if the levels are rising. Rising levels indicate pregnancy.
If this too is negative then other causes of delayed or missed periods will have to be looked at such as ectopics (can give negative urine/HCG tests), stress whether emotional or physical, hypothyroidism, polycystic disease of the ovary, pituitary tumors secreting prolactin, anemia and poor diet, hormonal imbalance, unexplained or sudden weight gain or weight loss. You will need to consult a gynecologist for this. Take care!