Yes, they go through MANY phases.
So it might be.
Or, sometimes they are this way when they are sick, or teething, or experiencing 'separation anxiety' etc.
Usually, when they need you for some reason. And, also as they are changing developmentally.
So, the thing for you is, whether this is okay with you and him and Hubby. Or, do you want/need him to sleep back in his crib? Is it a "problem" or not? If not, then perhaps he can just co-sleep with you. Some say this is a 'bad' habit... others say it is no problem at all. As children naturally grow out of it. In some cultures, this is all they do. Kids grow up fine and are perfectly well adjusted.
I think most Moms can relate to this. But they all deal with it differently....depending on, if it is considered by you to be a "problem."
And yes, you are pregnant... this IS a major thing for a child. So, they can often react in different ways...such as being more clingy, or fussy or not. So, main thing is for you to not make it a 'bad' thing or scold him for it...but rather to help him feel safe and comfy and secure about the whole thing. His whole world as he knows it will change when baby comes home... so an 'eldest' child needs extra attentiveness at this time... and after the baby arrives. Remember, a young child does not have 'coping skills' to deal with changes or 'stress'...so we have to help them.. .and sometimes they just want and need to be closer to Mommy.
All the best,
Susan