I had gone though your question and understand your concerns.In older people their actual blood pressure(ie.actual pressure inside the blood vessels) is a little lesser than what we measure with the B.P apparatus..it is called osler's effect,so hypetension in the elderly should not be overtreated.
The sudden rise in B.P(187/90), over a few weeks may not be normal and warrents further investigations and information.
-Is she diabetic?
-Is she taking aspirin
or any other pain medication?
-Is she having any arthitis or pain anywhere else in the body?
-Is she able to pass urine normally(any burning sensation or difficulty /decreased urine)
-Do u find any difference in her breath smell?(ammoniacal, etc)
-Does she have this belching
If I were your doctor, considering your mothers age and the onset of symptoms
-I would recomend a kidney
function test: UREA & CREATININE
a problem with the kidney may cause the B.P to elevate and the increase in urea due to a problem with kidney can cause belching.
This is to be on the safer side, if no abnormality is detected then our way of treatment becomes easier.