She starts a new diet, admits she’s not eating the recommended amount of calories, and has symptoms of low blood sugar. Why would we consider pregnancy (when she’s on birth control) before we consider that she’s simply not eating enough?
I was once on what I would now call a starvation diet (then, I would’ve argued it was completely fine and healthy) and limited my carbs to basically nothing. A few days in, I felt like shit. I was dizzy, nauseous, and lightheaded. I even passed out at one point. When I finally ate, it just made me feel worse. It took a few hours for the feeling to go away. Once I started eating more over the next couple of days, the sick feeling went away completely.
I’m not saying pregnancy is impossible here, but if there hasn’t been a missed period, there’s no need to rush out and buy a pregnancy test just yet. And there’s no reason to suspect some serious medical issue yet either.
Give it a day or two and stop starving yourself. Chances are you’ll be fine. Do yourself a favor and educate yourself on healthy eating. I eat over 1500 calories per day and I am steadily losing weight. Calculate your TDEE, subtract 25%, and eat at least that much a day. Losing weight slow and steady by changing your eating habits is a much better idea than drastically cutting calories in an attempt to drop weight quickly. You’re more likely to keep it off if you learn healthy eating habits now. Crash diets are for shit and they’re usually very unhealthy.