Mixagrip has a warning side effects that just as you described. It's a medication from where I live and we have this website that explains the side effects of it.
https://www.k24klik.com/p/mixagrip-flu&batuk-capl-4s-strip-25s-8817
While there's no mention of caffeine, be wary to use it with alcohol though generally, you shouldn't drink caffeine either.