to answer your question, the egg can get fertalized for about 24 hours and the sperm can live in you for a few days, so it is possible to have sex on monday and get pregnant on a wednesday.
i really hope that if you are not pregnant that you sit back and look at this situation. being a good parent means that you put your child first and that includes waiting until you are physically, emotionally and financially able to support your child as best as you can. every girl wants to have a baby at your age, but you need to look at the reasons objectivly. are you lonely? do you crave love? do you want attention? what makes you desire this commitment? please know that a baby will always take more than they can give. a baby cannot make you a better person, but you can do that for your baby. a baby cannot make you smarter, healthier, happier, etc... you do these things yourself, a baby may just make you want them more, but it is up to you. a baby that is a tool in its mothers life to serve her happiness will not be a happy baby.
best of luck