What Causes Lowering Of Blood Pressure While On Amlodipine?

I am 59. I hve bp since 1999 and a doctor prescribed stamlo 10 which I have been taking until recently there was shortage of the drug and a pharmacy suggested amlodipine which he said is similar to stamlo 10. I started taking amlodipine but on 23rd January 2015, I was taken ill suddenly. Infact I collapsed and was rudhed to hospital and I was diagnised to have low bp. After I was discharged, I was prescribed asomex 2.5. Dr. Can you tell me what caused my condition and whether asomex is good for me. My bp now ranges between 120/80 to 154/85. Thanks . Kamba