Cat Pooping outside of the litter box

My kitty is litter box trained. She was an indoor cat for about a month when I first got her and there were no issues with her using the litter box. Then she was a strictly outdoor cat (she could go through an open window into the garage when she wanted, but never came into the house). Now I've moved into an apartment and have made her a strictly indoor cat (the only outside time she gets is on the balcony). Since moving indoors she has started to poop over the edge of the litter box. She stands in the litter box with her front paws in the litter, hind legs on the edge and poops over the side onto the floor next to the box. I got a covered litter box and this solved the problem for about a week, but she has now discovered that she can simply stick her butt out of the box and poo onto the floor that way. Since doing this she has also started to pee with her butt sticking out of the door (pee was never a problem before). I think it may be behavioral? Just want to figure out a way to make her more comfortable in her new surroundings. Here is a little more about her.

- Sadie is 11 years 5 months old.
- Overall good health
- dewormed last week, she picked something up from eating a bird outdoors possibly, but is now fine.
- prior to me getting her she had broken a rib. It was never set properly so now it sticks out, you can feel it but it doesn't seem to bother her at all or cause her pain.
- she is very active, playful and loves to cuddle and get pats (in typical areas such as under the chin and butt scratches)

- I have used the same cat litter (horse bedding as a cheaper alternative to feline pine litter) for a few years now

- I recently put another kitty down, to whom I was more emotionally attached. I think she notices my sadness about this and maybe feels jealous

Are there any remedies/Bach flowers and you would recommend in particular for this? Am I correct in assuming that should be treating emotional rather than physical reasons for pooping outside of the box?

Any suggestions would be helpful. If you need any more information about her/me please ask. Thanks so much.