@Louisex21 you poor thing
I went through equally rough first few weeks too and by week 7 I went from 20.7 to 16.8. I could hardly walk without legs cramping, vision was going and hair thinning. Thankfully I could start normal food week 8 and it was fine...I could hardly stomach soft/purée food and suvived on very little. The milk allergy I developed post op didn't help as I couldn't have the protein drinks etc. I did discover pea protein however that is dairy free to add to bouillon broth drinks.
Once I started the vitamins with food I was ok within 5 days and have had no hiccups since hopefully never. Weight loss has slowed right down since but would prefer slower weight loss rather go through those 7 weeks again...I thought I was never going to be able to eat food...I was worried I was going to go blind!
It still is early days for you but keep in contact with Dr. Tbh you should not exercise for first 12 weeks but not at all if not maintaining some sort of nutrition. I found mini hummus with cucumber good and peanut butter mashed with banana a life saver at those times! Hope you start to get better day by day.