Wonky blood glucose readings, is there such a thing as a delayed spike?

So I've begun monitoring my blood glucose recently after a family member was diagnosed. Hba1c result hasn't come back yet, but we know my fasting glucose is usually normal, sometimes slightly high. However in the downtime I've been asked to monitor my post meal sugar. The results have baffled my doctor and even the diabetes UK helpline. I thought I could try asking here as people may have experienced this too.

Basically after the 2 hour mark, the results are on the higher side of normal. However after 3-4 hours, sometimes 5, it will suddenly rocket up again by quite a few mmol. This is usually when I start getting symptoms, and that worries me. I realise this is a medical question better suited to professionals, but I've exhausted all my local places to query so I hope this is OK.