Is There Any Possibility Of Getting Pregnant With Negative Pregnancy Test?

False Negative
Dear doctor. For the past month, i have been showings signs of pregnancy. Overly tired, food cravings (especially milk), my breasts are tender, the area around my nipples has got darker and there is a white substance coming out of my nipples and i have put on weight in a sense that my stomach or tummy has got rounder, Emotionally i am a ticking time bomb and i dont have the patience i normally would. I am showing most of the signs of pregnancy. So far i have done 3 pregnancy tests, and they have been at least a week a part and all done when i go to the toilet in the morning. 2 were invalid and one was negative. My question is, are false negatives common, is it likely that the tests were wrong? And if i am not pregnant, why am i showing most of the signs of pregnancy and what else could it be? Thank you, Sarah...