From Gazzaniga et al.’s Psychological Sciences:
When memories for past events are retrieved, those memories can be affected by new circumstances, so that the newly reconsolidated memories may differ from their original versions.
This happens, or can happen, chemically. Remembering a thing or event is done via the same neurons that captured and consolidated that memory in the first place. What of your memories is from that initial firing and what is from subsequent recollection-directed refirings is difficult if not impossible to discern.