This is due to when this contact is converted you lead is not converted thats why you are not getting any records.
Due to this you need to use Lead afterupdate trigger.
Lead conversion order of execution
Account Before (Fires based on lead settings)
Account After
Contact Before (Fires based on lead settings)
Contact After
Opportunity Before (Fires based on lead settings)
Opportunity After (OCR's not available)
Lead Before
Lead After (OCR's available)