Leaking nappy when baby wearing

[FEJ2016]

Hi all,LO is 3 weeks and we've been out for a walk with her in the material sling 3 times. Each time she has gone in with a clean disposable nappy, walks have been about 90 minutes and when we get back there is poo literally everywhere. I have to wash the child, the sling, my clothes everything! I thought the first was a one off but this has happened every time so far.
Does this happen to anyone else and what is the solution? She loves it in the sling and I love baby wearing but this isn't sustainable!

Try a different nappy? Maybe she needs the next size up?

I double nappy my DS at night time, could you try this?

What size nappy is she in? The wrong size nappy (usually too small) is the first thing to discount when a nappy is leaking.

Thank you! She is 9lbs and in first size nappies. I'm off to get some bigger ones tomorrow! :)