pregnancy possibility?

I don't know what is going on with me... I've had two periods that were late. First by 6 days then by 8 days. both times I barely bled (dark brown) then I had heavy bright red then the next day was dark brown again then it was done. This has happened with both periods. I am used to 6 to 7 day periods. a few days again (2 days before my fertile days) I had a bit of brown discharge but that was all. I've had sore boobs throughout besides this last week and had n ausia very faintly hit me every once in a while and during physical activity. but that's not even the weird part. I've been taking pregnancy tests and all have come up negative. so what do you think is going on???