Pregnant at 6 months PP

My boy will be 6 months next week, and I just found out I'm pregnant! I was actually about to have carpal tunnel surgery, like, I was already in my gown, ready to be wheeled back, and the nurse said "how old is your son?" Once we realized my hCG levels should not be giving a positive pregnancy test, she did another one just to be sure. I got a blood test, and it's positive, too! I have an ultrasound on Monday. I have no IDEA how far along I am, as I haven't had a period yet. While this is welcomed (we obviously weren't being careful) I was expecting it so soon. I'm happy to be done after this (we only want 2) but I'm terrified!!!! Give me words of encouragement. I think they'll be about 14 months apart. My boy is SO well behaved, and sleeps through the night knocking on wood, here. I know we aren't going to get this lucky twice, and that also stresses me out. What are your experiences?!