This hasn't happened to me yet, but I know it's only a matter of time so I'm interested to hear the advice of others on this thread. (Coincidentally we started at around the same time and we've lost about the same amount of weight so far; losing a stone in a month is great but it's bound to slow down...).
If I were you I might try intermittent fasting or calorie cycling (alternating high- and low-cal days, where the former are the ones you do your serious workouts). Exercise-wise, just try something you haven't before: HIIT, long-distance running, weight training, whatever. If you're now 13lbs lighter and you run, try running with a 13lb backpack :-)
Failing that, maybe try taking a week or two off, eating at maintenance and doing a lot less exercise. Your body might stop hoarding those calories quite so much.