why is it negative?

Began sore breasts, now the are just swollen

Fatigued

Very nauseous 

Certain smells nauseate me

Low back aches

Bloating

Hungry but everything just doesn't sound good to eat

5 days before my period was due I had what seemed like a period but a very light and abnormal period. It began spotting then would stop and then bleed a little heavy then stop and spot again. This went on for 4 days. I've taken 4 pregnancy tests all negative, except the very first test I took. I figured since that one was digital it may have been defective (seeing how it took 15 min to give me a result) I am feeling very nauseous and fatigued and currently 21 DPO. My question is, is it possible it could be symptoms of something else other than pregnancy, or possibly pregnant and not being detected?