Relax. You'll get there. This isn't a weight-loss program, but a health plan. Everyone transitions differently, and some people take more than a month just to get over the low-carb flu. If you've only been fully primal for 3 weeks, I think you should give it some more time. It can take a long time to heal chronic problems.
However, you should also not just weigh yourself once, but also take measurements of your waist, hips, thighs, chest, neck, and upper arms, at least. There is a possibility that what you consider "naturally slim" was under-muscled, and your body is correcting that now.
You should also consider that "fully primal" may include foods that you personally are not tolerant of or may be overdoing, such as nightshades, caffeine, dairy, or nuts. Lack of sleep and high stress can both prevent weight loss. There's a lot going on, but it's probably too soon to tell. Stick with it for another month or so, then start making changes one at a time if you feel you need to.