Pregnancy symptoms but not pregnant?

My period has already come and gone (next one starts on the 13th of may), i took an hpt test on thursday and it was negative. My husband and I have literally been intimate once (after a LATE starting, heavy ten day period) since my period stopped (last Sunday) because I've been feeling so crappy. so Its pretty set in stone that I'm not pregnant, and I don't Want to be because I'm getting a breast augmentation this month, and we aren't even close to ready for a third child. What can cause me to have so many pregnancy signs (fatigue, constant nausea, headache, sharp menstrual like cramps, breaking out) when I'm not? I do plan on seeing my doctor. Something isn't right and it's driving me crazy.