Hello. Thank you for contacting HCM. I have gone through your question and I will try my best to give you a satisfactory answer. The most fertile days of a woman is around day 10 to day 17 approximately. If you had unprotected sex on 27th july I would suggest you take a pregnancy test to rule out pregnancy. Being pregnant can lead to nausea, vomiting and lethargy in some women. Apart from that there can be other reasons for your lethargy....the most common being iron deficiency anemia. I would suggest you consult your local physician after your pregnancy test for complete clinical evaluation and further course of treatment. I hope this answers your question. Please feel free to contact HCM again if you have any concerns. Wishing you good health.