Inconsolable crying at night 12 weeks

DS coming up to 12 weeks, although still wakes every 1.5-2 hours at night & would generally nurse back to sleep without a peep after around 20-40 minutrs, the last week he's started waking & just crying, really tearful crying. Nothing has changed at night, still nurse lying down in bed, dark room etc, but I can't work out why it's all of a sudden become so difficult to get him back to sleep at night!

I'm starting to loose the plot as he's a terrible daytime napper & fighting him to sleep all night as well now is getting me down.

Any suggestions or ideas?? I've tried swaddling on and off (hot weather) but makes no difference