As far as I know there is no evidence to suggest that these Postcoital Headaches are linked to or are precursors for imminent serious problems such as subarachnoid haemorrhages.
I suspect (but have no proof) that based on their origin and the fact that they are supposed to respond well to the Indomethacin (which is an anti-inflammatory) that their origins lay in the changed oxygen/carbondioxide balance which may occur during 'the deed' and impending orgasm, similar changes can happen when people hyperventilate and give migraine type headaches.
I leave it to you to determine which way you tackle this problem!
FD