Confused about super late period

I am having serious mood swings, crying all the time, nauseous in the morning and my period is now officially 8 days late. I have take 3 pregnancy tests, all negative. Also my boobs are not sore which I know is a big sign of pregnancy. We have been TTC for 6 months now and my period is always spot on with its timing. I know that stress can be a factor in late period last but I'm not too stressed and there hasn't been a lot of big changes in my lifestyle. Anyone have any advice or comments??