How long have you been with your boyfriend? Are you going to college or plan to go to college(or any other continued education)? Do you have a stable income/job? Would you be able to support a baby on your own if your boyfriend leaves you?
Sorry to ask so many questions, but there is a lot to think about before bringing a baby into the world. Crunch the numbers in your head to see if you can afford a baby(diapers, baby food, health insurance, food, rent, gas, car insurance, etc) because it is all very expensive. You are 19 years old right now and still have a lot ahead of you to experience.
Symptoms are unreliable to go off of and if you still think you may be pregnant, take a test at least 2 weeks after having sex.
If you're positive of the day you ovulated, an you had sex four days after, there's no way you could get pregnant. If you were taking a guess on when you ovulated based on when you should expect your period, its possible that you didn't ovulate when you thought.
Well being that sometimes you can ovulate later than you think I'd say there's always a chance. Wait a week and test again! Hopefully you get the result you want!
Your chances aren't lower after ovulation, they're non-existent. A female can only get pregnant during ovulation or the week leading up to it, and the egg is only viable for 12-24 hours.