Being off work for pregnancy related illness

Hi, just wondering if anyone can help.

Im 30+5 weeks pregnant. I suffer from severe sickness and nausea. Ive been quite a lot this pregnancy but ive either had a sicknote or been used my holidays. 

My working hours are 3am - 12pm and i work in a factory, the job i do isnt difficult but when i havent been sleeping well so working while having been awake for over 24hrs without sleep makes me sickness even worse so its very hard for me to work. 

To get to the point.. 

Ive been off work since the 03 june 2019 and i called my doctors to get a sicknote and they said my doctor couldnt call me until today which i asked weather the sicknote would be backdated and she said mostlikely.. anyway, fast forward to today i spoke to my doctor and because ive been off already with my pregnancy related illness he had no problem in giving me a sicknote but said because he's only spoken to me about it today he couldnt back date it. Now i dont know what to say to my work because ive missed two weeks off work. Ive got a sicknote now to cover me so with they just put it down as unpaid sickleave or will they give me a disciplinary? 

I also dont know how to write it in an email to my work hr so what would i put? If anyone could help id really apreciate it