What Causes Headaches, Dizziness And Tiredness?

Since Christmas i have been feeling very unwell, constantly tired, randomly feeling sick, headachs, dizzieness etc, i am not pregnant and have been tests for heart conditions and have blood tests done all of which came back negative. i am now at a loss as to what is wrong with me but the constantly feeling ill and tired is beginning to make me feel very depressed. can you help?