P.O.M.
You need to check with a DAME that you are permitted to fly (and if not, how long you have to leave it before flying) after taking Imigran, even if you do not notice the any effect. But I'd have thought that not flying for at least 24 hours is sensible, not just because of Imigran but because of why you had to take it.
FYI, CASA's policy on migraine is to assess each case individually, according to their aviation medical manual.