So Exhausted And Weak All The Time, Any Idea What Could be wrong?

hey. i have a doctor appointment scheduled next week and a biopsy scheduled to check my thyroid also in a couple weeks. but i'm just really worried, and was wondering what people thought could be going on with me. i've been so sick feeling in the last month or so. i sleep all night long, and wake up in the morning still so drop dead tired. all i ever want to do is sleep. i'm so groggy and tired, and weak all the time. my body is so drained. i have no energy. my heart rate always feels so rapid. i have a hard time breathing at times. i have some chest pains. i'm dizzy. feel like throwing up a lot. i'm so scared i might have cancer or something serious. i don't know. any one have any idea what may be going on. with me. i have two kids, so watching them all day long is so hard on me lately, because of how sick i feel all the tiem.