Late period, early symptoms?

My period is late by 12 days. My SO and I are sexually active and use protection (we check before and during for any leaks and tears). We had intercourse 11/13 (last day of bleeding) then again 12/09 and 12/17. My period was expected to come on 12/16 but it has not and my stomach hasn't been feeling well lately - lots of constipation, bloating, pressures. I've taken a test and it was negative. I went to my gyn and she could not detect anything. Is it too early or has anyone else experienced the same thing?