What Causes Light Headedness And Nausea?

Hi im a 26 year old mother of two a 7 yr old and a 5 yr old. For the past 2 weeks ive been very nauseated light headed had tender breast and gained some weigh. I took 2 pregnancy test and they came iut negative. So today afyer 15 days laye my period is back but im still light headed and experiance nausea. What could this be?