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Welcome to Just Answer and thanks for your question.
While some numbness and tingling (all over) can be a rare side effect of flecainide, pain isn't.
The distribution of the pain makes me think of pressure on the spinal nerves. This may be made worse by sleeping position, for instance, or anything that aggravates the nerves. Here is a diagram that shows the distribution of the nerves from the spine. The area you describe would be from L1-L4.
An orthopedist would be the specialist to see, although your primary can order a CT for this.
Please let me know if you additional questions.