Hello dear, The early morning pill or the emergency pill has a high level of hormonal content. Intake of such a pill causes hormonal imbalance in the body. The delay in period can be attributed to that. Sometimes the periods can be delayed by even more than a month. The fact that the pregnancy test report has come negative indicate that the pill has done its role & you are not pregnant. So, there is no need to worry about it. In absence of any trauma/injury, the back pain can be attributed to fatigue or inadequate hydration status & electrolyte balance in the body. Though symptomatic relief can be obtained with pain killers, their regular use is not advised due to side effects like peptic ulcer, etc. So, kindly consult your doctor & get a proper clinical examination done. Investigations like Complete blood count, estimation of blood pressure & blood sugar levels will be helpful in this regard. Wishing you a good health. Take care.