Totally normal! :-)
My son was easy-going and never had separation anxiety for the first 17 months of his little life. I thought he must just be a laid back child until he - almost overnight - started just freaking out whenever I tried to leave him in a nursery at church (which he had LOVED for the last year and a half!) or even if we went to a playdate where I was with him, if we went to restaurants, doctor, etc., and especially if people came over. He turned into a different child whenever he felt threatened - he would head-butt and thrash and just scream.
It got so bad that I actually broke down in tears at his 18-month appt. and asked the pediatrician what was wrong with him! His answer: "Oh! He's being 2!" I said, "He isn't 2 yet!" He said, "He's just practicing!"
And you know what? He was right! I was 3 months pregnant with our second child when this started and was terrified he would kick me or that I wouldn't be able to physically handle him for much longer (I couldn't "make" him get in his car seat so had to alter a few plans!) and I went through MANY sleepless nights wondering if he had autism or anxiety or countless other things you hear about. I counted on my husband's help and it was honestly a VERY long 6 months with dealing with him and the pregnancy.
But then a strange thing happened. He actually turned TWO! He got a sister for his birthday and when I came home from the hospital, he was a different kid. I think part of it was just shock from his household changing and part of it was mellowing out with age. By then he could say a little more and communicate so I could explain in simple terms what we were about to do so it didn't freak him out so much. We didn't have a birthday party for him, but we DID start having 1-2 people over at a time for a play date, then 3-4 people, and so on and now if he hears a doorbell he yells, "Friends!!" and races to see who is visiting him! :-)
Sorry this is long, but I just want to let you know that I've been where you are! Your son senses that things are changing with you even if you're not showing and especially if you are! He's also going through a normal behavior cycle and it will (eventually!) get better. The best thing we did for our son was give him a sibling. Beyond that, we never - if we could help it - forced him into situations that made him uncomfortable. We had LOTS of people tell us that it would "make him stronger" to just force him to go to the nursery or out to crowded places, but we ignored them and realized that our son was trusting us to protect him and to slowly help him learn what to do in these situations.
Best of luck with the pregnancy and even if it takes a few months, you'll get your little boy back even better than he was before! :-)