CONFUSED

My period is 3 days late so I took a First Response and it came back negative! I'm not sure what to think other than I am not pregnant right? BUT my nipples have been so sore! As well as feeling super tired.. my husband and I have been trying for awhile, I've been through this constant testing obsession before, I've just never actually thought I was pregnant like I do now, also hadn't missed my period. I keep thinking I'm going to get it, but every time I go to check it's just thick white discharge.