I think the culprit might be Verapamil which I'm taking to control cluster headaches caused by a sporting head injury some years ago. Of course the information leaflet supplied with the tablets did not mention water retention as a possible side effect but the timings do coincide.
I'd just like to thank you for running this web site which is a great resource for information in plain english on my condition. Knowing about it and how to deal with it makes such a big difference.
I have a brain injury from encephalitis a car accident and the best thing I have found for the headaches is low dose Benadryl (children's dose of 5 ml every 4-6 hours) and an amino acid called L-Histidine which helps with inflammation. Taking a lower dose of Benadryl keeps you from getting sleepy.