I had the flu injection this week and apart from a sore arm, no problems.
I only take one 500 mg Metformin daily and have never had the side effect Patch so "subtly" refers to, more exactly the opposite, they cause me to have mild constipation!
I know it's the Metformin because if I stop for a couple of days everything is back to normal.
Last year I had the flu injection which included something for Swine Flu or something close to it, that made me feel like I had a cold but without the runny nose etc. just tired and a headache for about 2 days.
So like Patch says, probably the flu injection. Whatever, it's a million times better than getting the flu!
H