Hello there, I can sort of understand where you are coming from, seeing that I do sports like swimming as well. It can be really hard to get our energy up when we are tired and sore, and especially since you are still recovering. I would really try to save up your energy as much as possible today. Take a nap, lie on the couch, just rest. I'm sure your family will understand, especially if you tell them. Try eating foods that will give you energy today, without weighing you down as well. Fruits, especially bananas are supposed to be good for that. There are also energy bars that you may want to try. I tried to look for a good link to a list of energizing foods, but the lists tend to be muddled. You may want to try just Googling "energizing foods" or something similar, and take a look at that. Also, make sure you drink lots of water today. If you're dehydrated, that can really make you tired.
In terms of being sore, that's the worst! I've been told some light running can help, because it will loosen you up and help flush out the chemical buildup that causes you to be sore. It's helped me in the past, that's for sure. Just make sure you don't do so much you make yourself tired for later! Maybe some easy stretching as well. And actually, bananas are supposed to help for this as well, because of the minerals in them.
Also, try to believe in yourself before you swim. We can psych ourselves out so much, just by saying that we really can't do this. You can. It might be hard, but you'll get through it. Your team probably doesn't expect you to swim as fast as the other girl, because you aren't used to this event. Don't worry about what they think, just concentrate on yourself, and doing your best. That is all they can ask.
Good luck!