Unfortunately it could be so many things. First off is to consider overthinking a little bit, if you feel your memory performance otherwise is acceptable most of the time. As the reason for why this happens it could be a great many things. Clean up your sleep schedule and quality first. Then maybe check and work on the diet. Vitamin deficiencies can cause all kinds of subtle trouble. Next the obligatory exercise disclaimer as it will improve functioning. It can be frustrating to deal with the human body as many things aren’t as clear cut.