Yeah, stress leading up to engagement photos is pretty normal.
The important thing to remember is that these arent mission-critical pictures. If your outfit is awful or your hair looks silly or you flat-out hate your photographer then you can just redo them and adjust before the next round of engagement pictures (or even fire your photographer altogether if need be). However, if you are stressed, and you feel stressed on the day of the pictures, you will LOOK stressed in them as well. Try to relax and have fun. Engagement pictures are about more than just getting a nice portrait, it should be a romantic bonding experience for you & your FH. A nice moment to document the fresh love of an engaged couple.
Im sure theyll turn out beautifully.