There are many reasons why your BP goes up. It may or may not be related to your pregnancy. It may be your lifestyle (though I know you said you're trying to lose weight), heredity, or some underlying reasons.
I started with a low dose of BP medication too - 100 mg of metoprolol. Then after a week, my doctor increased it to 150mg. Now my BP is controlled at 110 - 120 / 70 - 80 depending on the time of day, my activities and the food that i eat.
I was diagnosed with Pre-E on my 35th week of pregnancy. Now I am on my 9th week PP and I am taking BP medications. My current BMI is 26, which is considered overweight. I used to be within the normal range before the pregnancy. I gained 50lbs during my pregnancy! and I lost 25 lbs since delivery. I am still hopeful that I will be off medications if I just change my lifestyle. As I did, I am sure that most of you have done exhaustive research on the internet on how to lower your blood pressure. What I found to be really true was that after eating food high in sodium, my BP would go up to 130/90 while my average is 110/70 (all this with medication). Another thing that I found out to be really true is that exercise lowers your blood pressure immediately after you cooled down. Just yesterday, around 4pm, my BP was at 120/80, then I went out for a 30-minute walk. I came back and measured my BP immediately to check how much it went up due to a workout, it was at 126/86 (which is expected when you're working out). Then I cooled down and rested for 5 minutes, checked my BP and it was at 110/74. It was awesome. Now I feel hopeful to be off the meds soon for as long as I maintain a healthy lifestyle. Also, I don't put salt or any seasoning with sodium on my food. I eat mostly fruits and vegetables high in potassium and I use brown rice. Here are some websites I found useful: http://www.all4naturalhealth.com/exerci ... ssure.html
and http://www.medicinenet.com/script/main/ ... ekey=23715
Just want to share this info hoping that it will be of help.
Disclaimer: this is based on my personal experience, not an expert's advice.