Could Switching From Nitrocontin To Imdur Cause Low Blood Pressure And Headaches?

Hi! My dad has been taking Nitrocontin 2.6 with a couple of other heart medications for the last several months. For the last one week, he took Imdur 30mg instead of Nitrocontin 2.6mg and he said he has been experiencing low blood pressure and a headache time to time too. In the prescription doctor wrote down to use either Imdur 30mg or Nitrocontin 2.6mg. Is Imdur the reason for this?