Hello. I Am 25 Year Old Woman. I Am A

Hello. I am 25 year old woman. I am a day a way from being a week late for my period and I'm wondering what is going on. My last several periods have ranged from 31 to 37 days since the previous period started with the average being 34. It's now been 40 days since my last period. I have taken three pregnancy tests all being negative. One on the 36th day in the evening, then one in the morning on the 37th, then finally one this morning (the 40th day). As I said all negative. I'm wondering what is going on...can you help me?