Ever since I woke up this morning I have been having extreme dizzy spells. Even when I sit down I get dizzy. I am about 21 weeks pregnant so I am worried that there may be an issue or if this is normal.

That usually indicates a blood pressure issue. It is hard to say why you're having the issue though. This is something I would consult a doctor about.

See a doctor... there are a number of things that can cause this... all of which you would need to know. Including issues with blood sugar levels as well as blood pressure..