Sorry I can't help much, Grace. I had the same dose of Lisinopril as you with no side-effects at all. 5mg per day is actually a very low dose. I think you need to see your GP again.
I've been low-carbing for my BG levels and to lose weight. I came off the Lisinopril just after Christmas because my BP is now okay.
I take magnesium citrate as a supplement, 150mg 3 times a day, which is supposed to help with BP. I believe it does. I get mine via Ebay or Amazon, called Magasorb, though there are other brands. The 'citrate' bit is important - it's the most easily absorbed. I used to take it as well as the Lisinopril. It will maybe help with the acidic stomach?
I'd Google it to check there are no interactions with any other meds you may be taking, and discuss it with your doctor to make sure. I believe the treatment for pre-eclampsia is hospital is large doses of magnesium, so its use for BP treatment is medically known.
Hope you feel better soon. Make sure you drink plenty of water.
Viv 8)