Yellow fever immunisation contains a "live" strain of the bug, so can tend to cause a few problems, (flu type symptoms).
It can be problematic in those with a compromised immune system, but experience tells us that effects are usually mild and short lived in those without.
According to my guidance, the effects are usually mild and short lived 5-10 days.
Who told you a month?
I'd ask your AME, that's why he can afford a Mercedes.