I'm sorry but I have to laugh, your sitter is 6 months pregnant, has a 1 year old of her own, your 5 month old son, and you don't think she should be too overwhelmed, lol? At six months along I was starting my daily nap routine so i find this funny ; )
Seriously, though, it sounds to me as if your son is having separation anxiety. At 8 weeks the world was still relatively new to him, but now he's had time to see what it's all about. At his age they know who mom and dad are, they're familiar with their home surroundings and the sights and sounds, their sleeping area, what they like and how they like it, etc. Things are naturally changing in his life, he's now taking solids for one, but some little ones don't take so well to changes in their lives, he may be a tad too young to take solids yet. Maybe he would just prefer that he stay home with you and dad all the time, impractical, yes, but he doesn't know that. Also, it doesn't matter if the child she previously cared for had no problems, he was a different child than your son, you can't compare them or gauge your son by his behavior. And check his mouth, is it warm, his gums swollen and/or red? Sound like he could be teething as well, he's the right age.
He's also at the age where he needs to be on the floor and moving around more, it's a natural part of his development and if he doesn't get to do that during the day he may be missing it since he gets to do that with you. Is he in a pack n play where he can move about or confined in his carseat? And, how does your sitter's 1 year old interact with him? She may be jealous, and even bothering him, not necessarily on purpose, but maybe taking things away from him, fussing over him and trying to hug or kiss him, things he doesn't care for. My little guy is 2.5, but when he was smaller he would cry if someone fussed with him too much or took something away, now he just yells and tells me what happened so i know what's wrong ; )
Are you planning on leaving your son with this sitter after she has her baby? If not, I would find your new sitter now, maybe one with another child his age so he'll have a built-in playmate. Not because this sitter is a bad one, I don't believe from what you've stated that she is. It simply sounds like she's trying to keep you informed of how he's doing during the day, and you're getting frustrated because he's not acting like he does with you. But if you're going to move him it's better you do it now. If he's going to stay with her ask what she does when he cries or fusses, and gently make suggestions, telling her what you do with him at home to see if that helps.
His behavior actually sounds very normal for his age, as much as we want our children to be happy all the time the reality is that it doesn't always happen.
{{Hugs}}