HiI I'm also 21 and got married in April (marriage is great, don't listen to the ones who say it's not)!
About the pregnancy thing, if you had sex 6 days ago, I highly doubt that it would show up on a pregnancy test this soon. But you could always try! A blood test shows up sooner and is more accurate though. Also, about your weight; if your body is able to become pregnant in the first place, thats a sign that your body is able to support a baby. Now I'm not saying it would be easy. Your doctor would give you a special diet and want you to gain more weight than the average pregnant woman. And you may have a period of bed rest also. Debra Messing from the show "Will and Grace" was very very thin when she got pregnant. She listened to her doctors, rested and gained the weight she needed to and had a healthy baby boy! There is no way you would be forced to have an abortion. No doctor can make that choice for you. Let us know how it goes ok?