Hi Dowris
I'm not an expert but have been led to understand that anything over 20, you should go to A/E. However, on reading the posts here recently (I've only started out on this stuff, I have to say) it seems folks can experience higher.
Hopefully, someone with more knowledge about it will post for you soon but, if you don't hear something soon, it might be worth phoning the on-call GP and asking his advise.
Good luck
J