Well first of all, you wouldnt notice a "stretching" feeling in your abdomen as early as 1-2 weeks after conception. You would notice that at around 12 weeks. The mild pains (if period-like) are a symptom of early pregnancy. The fatigue & nausea are also a sign of early pregnancy.
Heres the scoop on what you asked. Sperm can live in the vagina for up to 3 days. So it can take 3 days for the sperm to fertilize the egg. Once the egg is fertilized it goes through a process called implantation which is when the fertilized egg attaches itself to the uterus wall (thus making you officially pregnant). Implantation can take up to 10 days. After the egg successfully implants itself, your body will start to produce the pregnancy hormone (which you would test for). Once your body produces the pregnancy hormone, you will notice symptoms of pregnancy such as nausea, fatigue, sore/swollen breasts, etc.
The obvious is the missed period. You miss your period because the egg that would normally be released is implanted in your uterus holding the life of your little one.
Hope this helps you understand the process.
Educated in the Medical Field;