The vast majority of illnesses reported by patients as "flu" are not influenza at all but other viral illnesses, some of which can themselves be quite debilitating. I am not aware of a strain of influenza being about at the moment so this probably applies to the report of the OP.
Flu jabs are precisely tailored to the strain of influenza thought most likely to arrive in the UK over the coming winter. Frequently these anticipated strains do not in fact arrive to cause us a problem. There is little point in having a jab if you are young, fit and well and therefore not at serious risk if the anticipated strain does in fact arrive. The jab is there to protect those who through age or disability would be at serious risk if they caught flu. The jab will not protect you from the other viral illnesses which are endemic in this country and we all get from time to time. There are so many of these strains which reappear from time to time that it would be impossible produce a vaccine to protect against them all.