wa 'alaikumsalaam, what l've learnt max. time for darah nifas can be up to 60 days (according to mazhab As-Syafie -. and theres a method to specify whether its still nifas or haid. Hence, each person varies. U may want to follow /learn from Ustzh Nadia Hanim. Likewise, theres an exceptional too for breastfeeding mum on which we can bayar later on. Be it in a form of fidyah / & qadho' depending on the situation. wAllah a'lam.
Salam sis. You're still in nifas which is until 44 days so you cannot fast. Your fast yesterday is also invalid. It's ok. Islam is simple and you have a reason not to fast. Same goes for breastfeeding mothers. You can always bayar later on. For now have a good rest.