Hello. Could you please tell me if anyone has had this experience? I'm…

Hello. Could you please tell me if anyone has had this experience? I'm pregnant and need to do a bunch of tests. Today I went to the hospital to have blood drawn from a vein but they couldn't get it because I wasn't willing to go. In my eyes, it darkened and I felt bad. Tomorrow I'll try again to draw blood from a vein. I need to do many tests. Maybe someone knows what to eat and what to do so that the blood flows well.