Lulu41, you are not the only one! I also have a 5yo ds (and a 2yo dd).
I was horrible to both of them this morning because they were dawdling getting ready etc etc, and the stress at the time (and afterwards with the guilt) is awful. I've sometimes thought, oh no, this will be what my ds will remember when he looks back on his childhood, not the fun good times we have most of the time, but his mother yelling at him.
After a 'scene' I always try and apologise for shouting, and explain why I got so angry - I Always want to say goodbye at school with him in a happy mood, or to say goodnight in the same way. I tell him that I get tired too, or that we all have to work together in the mornings to leave on time. And we move on - until the next time...
Someone lent me a book recently called Raising Boys, and it was talking about this age of 5-6: they are still their mother's boys up to now, but are beginning to want to branch out and be like their dads - or any other male role model. Maybe as mothers we can feel this, we want them still to be our darling little boy, yet we want them to act their age. I don't know!
For you, being pg is a big strain, so don't be too hard on yrself. Also working (I work p/t). If you're really losing control, go into another room (the classic advice) - I did that this morning and said the F word about 50 times - then I caught sight of myself in a mirror, and had to laugh! (DH is away this week, and that's part of my problem at the mo'!)
Anyhow, take care, and enjoy the good times with your ds...