There are varying opinions about causes of new hypertension in pregnancy- of course the biggest fear is pre-eclampsia and although it is incredibly rare the PE strikes before 20 weeks there is some evidence the hypertension before this can be a precursor.
There is also some evidence that 'background' stress does not always cause higher BP so beware that this may not be the underlying cause- although it certainly doesn't help so do try to take steps to relax as much as possible and talk to your DH about this.
Officially hypertension in pregnancy ('pregnancy induced hypertension- PIH') starts when the diastolic (second figure) goes to 90 or above, BUT they also compare your BP with that of your booking appointment BP and if there is a substantial rise, even if not above 90, this can be a sign of problems. This happened with my second pregnancy and as I'd also had PE in my first pregnancy this was an early sign that things weren't too good.
PIH can be successfully treated with medication but as it is linked to pre-eclampsia additional monitoring may be required (regular BP & protein in urine checks, growth scans, blood flow scans).
Try to stay as relaxed as possible (I'm a big believer in antenatal yoga) and eat as healthily as possible. Do talk to your midwife if you're worried and familiarise yourself with all symptoms of pre-eclampsia just in case.
Fingers crossed it is just a 'blip' (which certainly do happen!) and your BP settles down or at least stays the same. Good luck xxx