Parents alway made me get flu shots and I would always have these heart flutters like my heart was skipping a beat. Also would play ball or go to the track and my heart would start beating like crazy like it was bout to burn out. I had to wear a heart monitor for awhile but they couldn't find anything.
Anyway all this stopped when I moved out for college and I didn't get them anymore cause didn't have insurance and wasn't going to the dr. Might be related or might not be but haven't had that issue in 10+ years which was probably the last time I had a flu shot.