I'd put another litterbox in a separate room too. Some kitties prefer not to share and if the current boxes are too close together one kitty might not like it. A general rule of thumb is one box per cat, plus one extra.

Is she going #2 in the box?

A vet check is definitely in order. Sometimes the only obvious signs of a UTI are excessive peeing. Not a lot in one spot, just little piddles because the urges come sooner than when the bladder is full.