For all she had learned in both her lives about the signs and symptoms, in the end it was-- of course-- the most worrying of signs that revealed the truth.
The first dizzy spell hit after a day of riding and going out to visit some of the nearby towns, as she was wont to do. It's been busy, and she'd neglected to bring a waterskin for the ride back. So, when the wooziness hits as she gets off her horse and she finds herself needing to support herself on a now very concerned-looking stable boy, she brushes it off to him and her guard as dehydration. It's a perfectly acceptable reason, one with past basis, so they all part ways perhaps a little shaken but largely comforted.
The second hits three days after that, when she's gets up too quickly off the ground after an early-morning bout of nausea drives her to the bathroom. It nearly gives Caspian a heart attack, but she tells him it must be some kind of bug-- she has been feeling a little off, lately-- and promises she'll take care.
Two more queasy and vertigo-filled days, and then she passes out while hurrying to a meeting with some Lord or another. The healer tells her with a tsk of her tongue when she awakens that it was likely due to the quickened pace and heat, and really, Her Majesty ought to know better than to expose herself to either when in her 'condition.'
That, of course, leads to Susan asking what exactly her 'condition' is, which has the healer beaming as gleefully as a Cat can and informing her that she was carrying the heir to the Narnian throne. She nearly passes out again at the news, recovers herself, and is asked by the healer if she wants her to tell the King when he gets back from his ride out to Littlehollow.
"No," Susan says, shutting her eyes and rubbing her temples unconsciously, "Don't tell His Majesty. I want to break the news myself."
Naturally, Aslan would will it that Caspian walk into the room-- early returning from his ride, but likely word travelled that she'd collapsed and he'd rushed back-- just as she speaks the words.