Is it possible for the pain to not start until about 17-18 weeks after conception?

I had an eptopic pregnany and it ruptured at 7 weeks. At 17-18 weeks the fetus is too big. Your tube would have burst long ago which causes internal bleeding and you would most likely not still be alive. You should check with your doctor to see what else it could be. One possibility is dehydration. Seriously. In my first pregnancy I was hospitalized with severe abdominal pain when I was about 20 weeks. I was checked for kidney stone, wasn't that. I was given fluids and once hydrated I was perfectly fine. Go to the doctor though if you haven't already.

If you have already had a scan, they would have determined the location of the baby. Most ectopics are in one of the tubes, but they can be elsewhere and any discomfort may not appear right away. This would be uncommon however. Certainly let your doctor know if you are having unusual pain as pultabator mentioned. It can be for various reasons and some are easy fixes. Always best to play it safe... that is what your doctor is there for.