High blood pressure???

At my 33,34,35 week appointments my blood pressure was high. 152/98, 161/102, and 157/100. But they make me lay in my left side and recheck it about 15 minutes later and it goes down. They told me to watch it at home and to go to labor and delivery if it ever gets high. I've been checking it and it's running high but then when I check it again about an hour later it's down. All my blood work was good and urine too. Anybody know what can cause this. Currently 36 weeks 2 days.