My bunny has soft yucky poo, why and what should I do for her?

She always had hard little pellets for poo and lately she has had large soft clumps of poo, very stinky and it keeps getting all over her back legs and around her rectum, what could be causing this? What can I do fer her? Also her diet consist of rabbit food, the same kind/brand always and she eats a small bowl of romaine lettuce each day, no treats. Also she is a few years old and she is a Mini Rex.