Great advice above, but let me add that stressing about the fact that you could be pregnant can actually delay your period if you are not pregnant. The best thing to do is calm down, relax and take another test in about a weeks time. If you keep testing after only 4 days of missing your period you are pretty much just wasting money on pregnancy tests. You have to wait a little while so the hCG can build up if you are pregnant.
Now the feeling of movement in your stomach....gas. Plain and simple. If you are pregnant, you cannot feel it at this stage.
When someone is really scared about being pregnant, some times they get their mind set that they are, before they even know for sure. Sometimes when people get that mind set, they can actually start experiencing symptoms (like sensitivity to smells), but its all in their head. They are causing this because they have set it in their mind that they are pregnant.
Yes, pregnancy is a possibility with sex, period. But the best thing to do is not jump to conclusions until you have a definite answer.
