I Have High Blood Pressure But Am On Medication. I

I have high blood pressure but am on medication. I am 64 years old, female, hypothyroid sense I was23. I've been experiencing lightheaded episodes,loss of appetite and weight loss. I just took my blood pressure. It was 83 over 54. Low. Now it is 106 over 66. I also have vascular problems (vein closures). What would cause my bP to fall?