Started work out program this month: advice?

I'm the mom of a 2 year old and I work full time, and thus I have degenerated into a middle age spread. I need to lose 25 pounds to get to an ideal weight. Got an elliptical and have been running 30 minutes a day 6 days a week for the last 3 weeks, and am eating 1500-1600 calories. I have lost five pounds so far. Recently, I added dumbbell weights to tone my arms, and I do 100 ab reps of various kinds to strengthen my core.

I want to do more weight work so I don't lose muscle mass. However, I have some important limitations:

1. I have an autoimmune disease that includes chronic rheumatoid arthritis and inflammation that affects all my major joints and some tendons and ligaments. This means that heavy weights and certain activities can lead to chronic inflammation that is really painful. Even when I was 20 lbs lighter and childless, I would have pain from heavy weights. I am hoping to work up to somewhat heavier weights, but right now, using only 3 pounds. My doctor approved the idea of working out and is pushing the idea of losing weight, though he doesn't have much to say about the specifics.

2. Because of my daycare and work schedules, all workouts have to be done at home. So, no circuit training at the gym, no Pilates classes, yoga, etc. Sadly. Can't acquire any more equipment either unless it's cheap... so it's just the free weights and elliptical.

3. I am doing curls and lifts with the weights. What else could I be doing that will tone up and maintain muscle mass at this calorie deficit level, but which will not offend my easily disturbed joints? Lunges and squats, for example, kill me. I know they are great for your glutes, but they are too painful for me even with no weights.

Thanks for any advice.