My kitten won't use the litterbox anymore?

My kitten MiMi is about 8 months old and has used the litterbox no problem. Then we got a new kitten about 2 months ago, Nala, and she's almost 5 months old and Nala has used the litterbox fine also. Then about a month ago MiMi had been pooping outside of the litterbox right in front of it or behind it. The litterbox has been in the same spot since we moved in our apartment. MiMi never made a mess of the litterbox but everyday I clean it there is litter everywhere, pee and poop outside of the litterbox. Both MiMi and Nala eat the same cat food, Purina Kitten Chow, and they get fresh water everyday. I live in a 3rd floor apartment so they are strictly indoors at all times. Both are happy, playful, and love each other to pieces. Why this sudden change of behavior? Also, just a little while ago after I cleaned the litterbox, MiMi pooped outside of it. Everytime she does this she comes and sits in front of me until I go in to look. When I was cleaning it up there was a little blood clot. What could this be and is it something really serious? Going to the vet is out of the question right now. I have yet to pay half my bills and Nala's last vet bill hit my wallet hard because she was sick. We adopted both kittens from the spca so they are spayed already. Nala was just 2 months old when she was spayed and MiMi was 3 months. So their definitely not in heat. Please help if you can, thanks in advance!