What Causes Severe Pain In The Feet While Suffering From Diabetes?

My husband has been having shooting pain in his feet especially at night when resting. He has had many sonar tests, ulta sound but found nothing he is not diabetic. His feet are light purple in color. Some swelling as well. He does have heart problems and has had a triple by pass 8 years ago. His dr recommended taking lyrica but my husband wants an answer to why this is happening we just don t know, and would like any other avenues or tests to explain the issue.