I get the flu vaccine every single year and have never had any problems with it. Legit the worst thing that can happen is you're vaccinated against a strain of the flu that turns out not to be the major strain affecting the public in a given year. There are a handful of strains of the flu and they vaccinate you for the strain they predict is going to be the most prevalent. Sometimes it's wrong, but there is no harm and i have been getting them 24 years in a row. I have never felt sluggish before, you might feel a bit sore at the injection site for a few hours, but it's literally nothing-- it hurts worse to peel the bandaid off of your arm hair. I would highly recommend you get it, especially this year given the dual threat of flu/corona, especially if you're older. Always trust your doctor and ask them if there are any potential side effects of what ever they prescribe you, so they know what you're willing to tolerate and they will help you work around that with alternative options/etc.