Concerns of Pre-E

My blood pressure has always been perfect, including during pregnancy so far but yesterday at my 20 week visit my BP was 150/90. I was calmer and more relaxed than any other appointment I've ever had so I was surprised to hear that. My doctor said that they start to worry about preeclampsia after 150/90 and that next time it it's high I'll have to start on medication to help. I'm overweight but I totally did not anticipate this possibly being an issue. I have had a feeling that I'll have my son early (I feel like just a week or two) but obviously with pre-e I know preterm labor and premature birth is an issue. So I'm kind of stressed out at the idea that my baby could come even sooner than I think. What are some contributing factors to high blood pressure? Are there things I can do to help prevent it?