I have hyptertension, you have to remember to breath! Are you holding your breathe too much particularly when you lift the weight up and trying to go way over your limit? Meaning, you should be able to do at least 8 reps in good form, if you are straining too the point you are losing form before 8 reps then you are not doing it right. Are you drinking enough water?
Just a couple thoughts, but man..you need to check with your doctor, you may need to have your hypertension controlled by medication (if it is not already) before you try to do real heavy lifting. You need to check with your doctor....