Hello I have had high blood pressure for 2 years now and taking medication before and now in pregnancy.
The midwife should have explained better but 124/92 is not extremly high so i wouldnt worry. The top value (systolic) is ok as this should be about 125 the bottom one is slightly hish (distolic) average should really be 80
140/90 is when you need to go to hospital and as yours is not that yet do not worry.
What should happen now is you wil be monitored more than in a normal pregnancy by a consultant. I get seen every two weeks to have blood pressure checked and have had to start taking 2 pills a day as it is 132/90 most days.
But dont worry bout taking pills they will not harm the baby and dont worry at moment bout blood pressure they will monitor it very closely. The only time you should worry is if your feet and hands swell up, blurred vision and protein in urine this could be pre eclampsia but again if you are monitored then its not a problem.
As to why you have it and why you might have had it before. There are lots of reasons, heriditary, stress, weight, other problems such as kidneys wtc, too much salt in diet, not enough exxercise or like me no reason at all!!! I have been having tests to find out why for 2 years and there is no reason whatsoever so i just take pills everyday and keep monitored.
Sorry bout long post hope its helped ease your worries and explain more about it Here are some sites that may help but definitely bring it up at next appointment and if you have any other questions dont hesitate to ask!