She's bored, that's all. Some kids need more attention than others. My DD was like that. That's because she has an extreme need of mental stimulation and not an ounce of creativity, do struggled with occupying herself.
Yes it is annoying at times, but the more you push the child away, the more they'll crave the attention.
You need to work out the right balance. It's gift likely that her dad has find appeasement in getting you to take over giving her attention. He might not have given her much to start with, so she sherished it.
You were very keen at first. It was new, you were making a good i.otession with your new boyfriend winning his heart.
Now you're in an official relationship, pregnant and she is suddenly getting in the way. She is suddenly not only rejected by her dad but you too. Double blow.
You need to dedicate some special time with her, as her dad, and then ensure that she has clear means to entertain herself with what she enjoys. She needs to feel at home in your house rather than a guest who needs entertaining.
Ultimately, it's your OH who needs to react because she is not happy, and if coming at the weekends is more upset than fun, she will indeed start to say she doesn't want to come at all and it will be too late then, after many ignored warnings.