I’m a bit late replying to your question but once (1990) I had a very severe hypo one morning and was taken to A&E. The nurse said to my Mum, who had gone with me, that I would suffer from a severe headache for the rest of the day due to the brain swell caused by the severity of the hypo. I couldn’t recall this but having a rather severe hypo yesterday (no A&E trip necessary) and subsequent almost migraine feeling the entire day my Mum mentioned the incident back in 1990. There’s gotta be truth in this as you have experienced it too. Perhaps many others have also. I’ve had type1 diabetes for 44 years but the headaches can be quite severe after a hypo; I hadn’t put 2&2 together, stupidly.