I wouldn't be too concerned at this point. Lots of strange feeling surface when starting Atkins. I on the other hand felt great for the first 3 or 4 months, but for the last 2 I have been very nauseated. I have posted this question a few times on this board and can't seem to find anyone else who has had this as an issue after induction. (My doctor can't figure out whats wrong either ) If it continues past the induction 2 weeks, maybe you just need more carbs to function, which isn't unheard of! The only thing that concerns me, is you say its in you ribcage, which is where your gallbladder is. A few rare people (especailly if You ate very low fat before) sometimes do have some gallbladder issues when switching to high fat (the gallbladder releases bile to counteract the fats ingested, sometimes it can get stones if it hasn't been working due to low fat and when you eat a lot of fat can have some problems.) If you still feel bad in a while go to the doctor and have them check you out!

Hope you feel better soon! Nausea is no fun!