Your feelings are completely natural! No one wants to find out that there is something different about their child that might make their life more difficult. When you get news like that you grieve for the type of 'normal' life he won't have, you grieve because you fear life will be harder for him than you would like, you grieve because you know there will be things out of your control that you can't fix for him. It's tough. You also wonder how his diagnosis will affect your family, your other children, your lifestyle. You might criticize yourself for thinking these things because it feels selfish - it isn't. It is all normal and completely natural.
All of this is probably complicated right now by the fact that Down Syndrome is only a possibility. Not knowing is always harder than knowing, whatever the results.
I will say this, Down Syndrome is not all that difficult for a child or parent. There is of course a spectrum with some people being able to do and understand more than others. There are also some medical complications more likely to occur in individuals with Down Syndrome, but most are manageable. I have worked with many children (and adults) with Down Syndrome and have a few relatives with it. Their lives are different, but no less full. Furthermore, with regards to your other children and family life: I grew up with a little brother with a very rare chromosomal abnormality (trisomy 15). Our lives were different because of it, but I still had a great childhood and I think became a more patient, compassionate, and caring person because of my brother. My parents were always very open with me and involved me in my brothers care as much as possible. In fact, they encouraged me to play with him as though there was nothing different about him, this helped him learn because as a child I saw no reason for him not to walk, talk, etc so I just kept on working with him until he got it (I am almost 3 years older than him).
If it turns out that your son does have Down Syndrome, I suggest looking for some counseling or a support group. Hearing other people's stories and learning what to expect will make things a lot easier. You'll be able to plan and envision what life will be like.
All that said, I hope your baby is perfectly fine and that you get your results quickly. Don't beat yourself up about any feelings or thoughts you have. It takes time to process this kind of thing and the unknown is often scary.
He was such a cool kid! 
Last time I saw him was 10 years ago and he'd grown up into a very cool adult. 
