Yes take it slowly, I'm sure to lunges with dumb bells so those didn't affect me too much as I know they can be just as tough as with a barbell. Remember no matter where you stop stretch and stretch and stretch. This will help with some of the soreness, also jump into a nice warm bath with some sea salt, and massage any muscle you think will be sore the next day and get some of the latic acid moving and washing out like it's suppose too.
Now if your doing legs and your lower body hurts, make sure you got that core, as tight as you can without turning yourself inside out. But seriously sqeeze that stomach as tight as you can with even the slightest bit of stress to your lower back and if it still stressing, stop rewind and watch, see if you might be dong something a little different then Cathe. Even a simple lose muscle where one should be tight can cause you all sorts of problems, with the lower back. And if you still having trouble drop your weights, and try to work threw the exercise, the added weight may be pulling you out of alignment after you do one or two. As with my clients I put them in the right position they look like they got it, until they bring the weight back to the starting position and they don't got it any more. If this is what is happening and you got to be very very self conscious to know this, as you won't feel it right way, or maybe not at all, well at least not the day of the workout. Drop to a lower pound weight, and see if if you can do the right movement then. Also try to stand in front of a full length mirror or someone who has a good eye to detail and either look in the mirror while you do the exercise, or make them watch you and Cathe and compaire what your doing to what she's doing. Both work really good and getting you into the right position as you can be missing something really simple that you don't think about, which is the case 90% of the time. I am assuming you have no known back problems, that would make any of these exercises more difficult to do.
Good luck and do keep safe so you can continue to workout and enjoy the fun.
Kit