Congratulations on your successful loss and maintenance thus far!
The spectacular losses reported by first-time lowcarbers generally happens because up to the point of beginning Induction, they'd been eating a high-refined carb diet; also those with more weight to lose in the first place. Their initial losses are mostly water ... which isn't to say that it's not significant loss -- who needs all that extra water choking and clogging our cells? But, you have been lowcarbing for a long time, which means that YOUR body is in fine metabolic shape ..
... and doesn't have all that water weight to lose.
A possibility for the gain ... as Wa'il stated, it may be muscle tissue. Did you take measurements? Muscle takes up less space for the same weight as fat, so you will be leaner even though the scales show a net gain.
When you were maintaining on 50 carbs a day, were you also eating adequate protein and fat?? A lot of people mistakenly only focus on cutting out the carbs, and forget to replace those calories with something else. And now that you have cut the carbs back even further, your body is thinking "oh-oh, less food = starvation mode", and your metabolism gets slowed to conserve. Do try to eat lots of meat and protein, and bump up the fat intake with essential fats, olive and flax oil, fatty fish etc.
Exercise will help boost the metabolism as well.
Another suggestion might be to follow Induction to the letter as outlined in Atkins book. Use the menus in the book. Limit your carbs to vegetables and salads, and avoid the so-called hidden carb foods ... lowcarb bars, shakes, sugar-free treats. Drink buckets and buckets of water too. And weigh and measure the food. I've been lowcarbing for 14 months ... and find every now and then I need to get the measuring spoons and kitchen scale out ... my eyeball loses its acccuracy after a time.
These are just some ideas, maybe there's something helpful for you ..
.. Keep us posted ... we love success stories!
Doreen