Do let your GP know. It may be that your immune system has been activated. This happened a few weeks ago for me when I had the shingles booster. Although I have no recall of getting the vaccine in Feb

obviously the booster reminded my body to fight : after 5-6 hours I felt off colour, very cold, couldn't get warm not even under the electric blanket. The worse as the loud tinnitus and electric zaps up the back of my head, which kept me awake until the early hours. Then the headache began so up at 5.00 a.m. for Nurofen

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DH let me sleep in. I knew that the effect was the immune system kicking in so wasn't too worried. However, I won't have the flu booster this time as I haven't had 'flu since 1989. I am booked in for the Covid booster soon.
Get some liquids down you with Paracetamol. How is the injection site?