Perhaps the 1200-1300 calories/day you're eating are not enough. If you don't eat enough calories, your metabolism will slow down in order to preserve fat because there isn't enough food coming in.
Another reason is perhaps you aren't eating the right types of calories. If you're eating too much bread, potatoes, rice, etc... Those are simple carbs that are broken down quickly and if not used for energy right away, will be stored as fat for later use.
Fill your plate with veggies, protein and healthy fats...
Hi may not be much help but I know the feeling had my baby last year went from size 8 to a 14 my scale won't budge either I think sometimes it takes a while to get back to normal feel better I'm currently blaming a c section xx