I suffer menstrual migraine. Being put on decapeptyl injections soon for other issues but could this make my migraine worse?

Might or might not. Headache is listed as very common (>10%) side effect of Decapeptyl, but these are not necessarily migraines. Decapeptyl is going to change your hormonal cycles (that's why you're being put on it, I assume) which might themselves be responsible for your menstrual migraines. So you might have no change at all, might improve, or might get an additional form of headache. Good luck!