Do flecainide and diltiazem work together?

I certainly do not want to alarm you nor scare you at all.

There is a major interaction between both medications which means:

Major Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit.

Interactions between your drugs
Major
dilTIAZem flecainide
Applies to: diltiazem, flecainide

Using dilTIAZem together with flecainide can increase the effects of either medication. This can cause dizziness, weakness, chest pain, shortness of breath, fainting, an unusually fast or slow heartbeat, coma, slurred speech, and confusion. You may need a dose adjustment or need your blood pressure checked more often if you take both medications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.

Interaction report: https://www.drugs.com/interactions-check.php?drug_list=890-0,1094-0

BTW, to ease your mind, most likely your Dr. is aware of the potential interaction between both medications and has outweighed that the benefits are greater than the potential risks.

But anyway do contact your DR. asap.

All the best, masso