feeling dizzy... should I worry?

I am 23 weeks pregnant and in the last 4 weeks have nearly fainted 3 times. I have felt so dizzy and my fingers have tingled and my head has thumped and hurt so
Much. And I have felt sick. This has happened even when I have eaten a lot, drank plenty of water, and haven't been doing too much although each time it has been at work. I am starting to think it may be something more serious like a brain tumour. I have been to my doctor but blood pressure is fine... I'm starting to worry now 

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