As a memory researcher, my two cents is that being sick likely leads you to have fewer distinctive episodic memories encoded during the time you are sick because you are mostly staying in the same place (e.g., your bed) and enduring the same discomforting symptoms for a few days. So what you lay down is a schematic type of memory that kind of encompasses the whole period you are sick, but individual episodes within that period of, say, 2-6 days, are likely to be sensitive to rapid decay as they lack distinctive traces to support their reintegration when later you try to remember them.

Another element to consider is the fact that all memory is reconstructive, so we never remember a past episode exactly as it was encoded. Our current thoughts, feelings, and motivations color how we perceive past episodes. Given that sickness is generally undesirable, when we are not sick, we probably don't have the motivation to reconstruct memories of the time we were sick.