FYI, to compare "what it could be". Union rules here (country in scandinavia) forbid the employers to ask what is wrong, when employees call in sick. They can, after some time, if the employee still is sick, ask for a doctor to verify that the employee is indeed unable to come to work. The doctor though can only write a letter confirming (if that is the case) that indeed this employee is unable to attend work due to medical issues. No description is needed or required, not even an estimate of when they will be ok again.

Also, you get full sick pay while sick, medical care is free, and if the employer asks for a doctor confirmation, they are will have to pay for it (current price aprox €100)