Hello,
Welcome to the ehealthforum and I am really glad to help you out. Your concern is regarding the status of your pregnancy. Since your last menstrual period was on the 6th of December and you had unprotected intercourse around the 27th of December, chances of pregnancy are less likely. Pregnancy occurs only during ovulation days which occur around 10-14 days post periods. Your ovulation would have been around the 19th of December and the best timing of intercourse in order to conceive is day 10, 12, 14 and 16 (in case of late ovulation) of a 28 day cycle. So pregnancy can occur in your case if there was a late ovulation.
For confirming the status of pregnancy by urine for pregnancy test (UPT),the most accurate way to test for pregnancy is to do the urine for pregnancy test after 1 week of your missed periods date. You have gotten the negative serum HCG test also which in a way rules out pregnancy. This delaying of periods can be due to many reasons like stress and anxiety, malnutrition, depression, overexercising, sudden weight loss or gain (obesity), thyroid disorders and a hormonal imbalance due to polycystic ovarian syndrome(PCOS). I would suggest that you get these causes evaluated from your gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For an exact diagnosis, you are advised to consult your doctor. I sincerely hope that helps. Take care.