Kellie,
That is what fear does to us, makes us hostile. A question you might ask yourself is, "How is all of this decision-making working for me? No guidance from God who created me and loves beyond anything imagineable! He created you, and you are "fearfully and wonderfully made" and so is any baby created in or out of marriage; but yes, it is not God's best, nor His plan for your life. The Bible has an the answer, the right answer to any question in life. Unfortunately society has done a really good job taking God out of just about everything. I am not lecturing. I have been in a similar type situation, and others even more stressful and upsetting; so I am not talking in the wind. My decisions, before Christ, were really bad, with really bad consequences.
Just because you are 20 own your own home, is nothing, compared to making good choices about a life that might be created out of sex that has no committment. The morning after pill is just a form of "abortion". Check out the link below and my website please, all of it, especially page 2, page 3 might enlighten you to some truth as well.
From your circumstances, as far as just answering your concern, here is a site from yahoo answers with varying personal experiences regarding your ?, including one lady saying what her ob/gyn said.
https://answersrip.com/question/index?qid=20061003101244AAwyG89 Sounds like it varies a lot, from body to body, and none of us are the same. Chemicals are different in different bodies. I think there is certainly a possiblity you could get pregnant. Just remember, if so, this is already the beginning of new life that never was before and never will be again, a baby, not a thing, not a blob, not just a bunch of cells, but an actual new life whose DNA has never existed before, as unique and special as you are.
I recommend reading the book, "Fearfully and Wonderfully Made." I wish the very best for you, especially that you might invite Jesus into your life and allow Him, instead of you, to be Lord.
http://www.christianliferesources.com/article/the-abortion-pill-and-the-morning-after-pill-are-they-the-same-1023
http://www.christiansexgodsway.com