I am nauseous, extremely tired and major pain in my lower back. what could be wrong?

Need to examine: If you had sex since your last period and missed your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully. If you are not pregnant, it would require an in-person examination. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity at tolerated.