back ache, fatigue, headache and nausea?

Hi,
Welcome to ehealthforum,
If you have unprotected intercourse after your periods, you are closer to your fertile days or probable time of egg release, hence chances of pregnancy cannot be ruled out. Pre-cum, or pre-ejaculatory fluid is known to contain sperms in it. Sperms once released inside female reproductive tract can stay alive for upto 3-4 days. The egg once released from ovary can stay alive for 6-7 days.
Your current symptoms (low back pain, headache, nausea, increased sleeping, mood changes) could suggestive of possibility of early pregnancy.
Wait and watch for your next periods. Look out for development of any other new symptoms that could suggest possibility of early pregnancy. If your periods are more than 10 days late, or if you develop any other symptoms, you should consider visiting your doctor/gynecologist for further evaluation/examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). Drink plenty of water. Take adequate rest. Maintain healthy diet.
Take care.