Wait until your next period is due. If you miss it then try another pregnancy test, or see a doctor for a more accurate blood test.
Negative results when you're actually pregnant are fairly common. It could just be that it's too early to test, and that's why the test was negative. Some women don't test positive for months, even though they are in fact pregnant.
Could it be stress? Yes. It could be a lot of things, including pregnancy.
When you test make sure to use first morning urine, and follow the directions exactly.
Edit: Just want to add, there's a term used for people that use the pull out method. That term is "parents'. It's not a reliable form of birth control. There's plenty of sperm in pre-ejaculate to get you pregnant.
Any time you have sex, even if you're using protection, pregnancy is possible. So, if you get lucky this time and it turns out you're not pregnant, learn a lesson and stop having sex until you're ready to be a mother.