Off sick and references - please advise... !!!

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OK so I am currently signed off sick with work related stress. I am a teacher and actually resigned my post to finish in the summer as I was finding it too stressful. I registered with a teaching supply agency at the time with a view to doing supply teaching from this September onwards. However, my current headteacher didn't want me to leave so offered me reduced hours and a different role which I accepted. Unfortunately, this new role hasn't worked out as I hoped and I have ended up being signed off with stress. I have since resigned my post but won't actually leave until Xmas due to my contracted notice period. It is likely I will be signed off sick until then. Just by chance, this evening the supply agency I registered with in the summer have got in touch to see whether I would like to re register with them. I have said yes to this with a view to doing supply work from January. They have looked at my file and, because it has not been very long since I first registered, they don't actually need me to do anything. However, they have said they need an updated reference from my headteacher. Now I am worrying because I did not tell the supply agency that I was currently off sick, I just accepted to do supply work from January. I'm worried what my head is going to think when she gets a reference request from the supply agency while I am off sick. Does anyone have any advice? Should I get in touch to let me head know this has happened? Or just sit tight and wait to see what happens? I'm really getting worked up about it!