I'm sorry you're dealing with this; I know how frustrating and exhausting that can be. And by all means, talk to your pediatrician.
But most likely -- as in, 95%+ probability -- there's nothing wrong, it's nothing you did, nothing that happened, and nothing you can do. Kids at that age go through such rapid developmental stages, and a big part of that is recognizing themselves as a separate being from mom and dad and asserting their independence. This is all exciting, but it is also terrifying, and as we all know, fear can tend to manifest in dreams, or as a fear of the dark/monsters, or similar. It's all the brain's effort to distill that sort of existential angst into something that it can put a name and a face to, so that it can process it and "defeat" it. This can all be pushed along by the new baby, of course -- because the kid sees the new baby so attached to mom all the time, which brings home that the older kid is not so attached any more, which, again, part of the kid is happy with, and part of the kid is terrified by.
Of course, that doesn't solve the problem!! And I, alas, don't have any actual useful advice, because I was crap at managing this myself. But perhaps knowing that it's normal will at least ease some of that stress of looking for a cause. It's nothing you did, nothing that happened. And it's also not the kid acting up or being bad to get attention. It's just a child with limited verbal skills wanting more independence than child's brain is currently equipped to handle and needing to develop the self-confidence to know that child can handle it.