Ibuprofen once a month bad?

That shouldn't be bad at all if I'm understanding your question correctly. Also, if your cramps are really as bad as you say, then Ibuprofen might not even help and if that is the case, you should really see a doctor (Gynecologist if your cramps are the kind that I think you mean). If the Ibuprofen does help, then by all means it's totally fine!