Unexplained Dizziness

Hello, I am 25 weeks pregnant and have unexplained mild dizziness (feels like a head rush) for the last 3 weeks. My blood pressure is normal, blood sugar is within normal limits (I have gestational diabetes, hence test 5 times a day), my doctor ran all blood tests like iron, calcium, magnesium, liver profile, kidney profile etc. and all are within range. It mostly happens at the same time during the day and a couple of times at a different time. Each episode lasts 20-40 mins and gets better when I lie down. My doctor is also a bit surprised and trying to figure out possible reasons. My doctor said that since it is quite mild and goes away, there is nothing to worry about but he would ask his peers as well for their opinions. Has anyone experienced anything similar?