When I found it on the swanplant, it was in it's 3rd instar. Once in the caterpillar castle it ate milkweed leaves for about a day, then it stayed very still under a leaf for 48 hours. This indicated to me that it was in preparation to change it's skin.
This morning I found that it had slipped out of the old skin and is now in the 4th instar.
Monarch Caterpillars change their skin 5 times before forming their chrysalis. This allows them to grow bigger within a skin, before slipping it off in favour of another skin that will allow them to grow even bigger.
This caterpillar has just done this and is eating the old skin. It will have one more skin change before growing really fat and later forming its chrysalis. 🦋
(Please excuse the quality of the photos taken with my phone)