How old is Lexi? Did you just get her?

My cat, Snow, use to do this when she was a kitten. Lexi might not know where her litter box is.

Does she go in her litter box or just all over the house?

Here is some advice: you may have to train Lexi to use the litter box. Put the litter box where Lexi usually goes. Lexi will soon start to go in there. Another idea my mom use to do when my cat had the same problem was kinda crazy, but it did help. Whenever Snow was ready to go to the bathroom on the rug and furniture, she would pick her up and run to the litter box. She would put her in it. Snow soon learned she had to go in there not all over the house.

Do you have another cat?

Lexi maybe sprending her scent and/or marking her terratory. This may be the cause of this bad habit.