What your vet said about UTIs, constipation, and other difficulty voiding is very true. You said that she went in her box for a few days after the last vet visit and now she is at it again… she might be developing problems again. Take her back to the vet. I had a cat with a uric crystal problem and, whenever it kicked in, she did that. It was how I knew when she was having the problem. Be sure that your litterbox is kept absolutely clean. If it is (I suspect it is based on how much it looks like you care about your cat) try changing the litter pan itself and using a different type of litter… that might help break the association with pain that your cat has. Before anything else, though, I would take her back to the vet.