Is it normal to suddenly get dizzy spells when standing up after starting a new exercise routine?

I recently started a high-intensity interval training (HIIT) program a week ago, and for the past couple of days, I've been experiencing brief dizzy spells every time I stand up quickly. I'm staying hydrated and eating normally, so I'm really confused. Has anyone else encountered this, and what might be causing it? Any tips on how to manage or prevent this would be greatly appreciated!