Actually ectopics do often show up as a positive hpt - although typically the the hcg levels are lower.
And secondly you wouldn't neccessarily be in severe pain - I wasn't and I had a positive hpt.
The pain is caused when a fallopian tube ruptures, there may or may not be mild discomfort prior to that. However not all ectopics are in the fallopian tube (mine wasn't) and therefore do not neccessarily have that symptom.
[please make sure your information is correct before posting it as a statement of fact]
to the original poster why are you so concerned that the possible pregnancy may be ectopic?
The quickest way to find out is to see your doctor/gyn/planned parenthood and get a test done to find out if you are pregnant and if not why you don't have your period.