First of all dont believe ketosis strips - they are notoriously unreliable! What about the other signs of ketosis?
Anyway, lots of reasons to put on:
Totm water retention
Having sugar free goodies or other "frakenfood"
Too much salt (water retention)
Too little water (water retention)
Lots of exercise (water retention)
Eating an absolutely mind blowing number of calories (atkins says dont count calories not calories dont count)
Booze
Body just feels like it......
All of which says.....it's most likely water and will come off unless you have crab creep or indulging in sf stuff. Not sure if you are keeping a daily menu but suggest you either have a diary here or use something like MFP - then we can check for you
Dont panic, keep going, good luck!