No, there is no reason for concern.
While the creatinine is primarily checked as a reflection of kidney function, it can also be affected by a wide variety of other conditions. In particular, when someone has an acute illness that affects hydration, it can cause the creatinine to be falsely elevated. Most people that have a flu-like illness are not well hydrated, and they typically need to be drinking more fluids than usual to become well hydrated.
The eGFR is a calculated number that is based on the creatinine, so when the creatinine is falsely elevated, the eGFR will be falsely lower.
It is also true that we typically would only be concerned when the eGFR is below 60, but even the minor decrease in eGFR in these results will probably be normal if it were rechecked a few weeks after the flu like symptoms have resolved.
So, this is a common non-specific change that is seen during acute illnesses, including flu-like infections, and there is no reason for concern (other than taking the antibiotics that have been prescribed).
If I can provide any further information, please let me know.