It's been4 weeks since I had unprotected sex I've been feeling very run down belly is hurting every now and then my nipples are very saw and fragile to touch I've been feeling so tired an I've been getting huge head aches recently I should have been on my period 2 days ago, I know it's only to days. I took a pregnancy test 2 days ago and both of them said negative. Why do I feel pregnant if the tests say negative ?