Healthy weight loss rate??????

I'm new to this site and new to dieting period. I never knew how difficult it could be to loose weight until about 5 weeks ago when I decided I had let myself go far enough. My wife and I was having a discussion this morning while we were getting ready for work as to what a healthy weight loss rate was.

I started this journey on 2/21/11 and 5 weeks later I've dropped 17 pounds. I'm eating approximately 1200-1500 calories per day and I'm doing sit ups and push ups on a daily basis. I try to get some walks in here and there and I've started parking at the far end of the parking lot to at least get a little more of a walk into the office everyday. My wife says I'm loosing it to quickly. I don't feel as though I am because I'm feeling better and better as the weeks go by. If I didn't feel so good I might agree with her. What's everyones opinions? She's been on and off dieting for as long as I've known her which is about 13 years so I feel as though she knows what she's talking about. She's currently started weight watchers which works really well for her but I wanted to try it on my own. The biggest change I've made is all I drink is water and an occasional glass of orange juice. In the past I'd drink 2-20oz bottles of Coke a day and I'd go through a gallon to a gallon and a half of orange juice a week. Could this have been the cause of my weight gain? I also don't eat out for lunch everyday anymore, I bring a sandwich which tieds me over to dinner and we usually have a nice homecooked dinner now in the evenings. I don't feel as though I've changed a whole lot of what I do but yet she seems to think the weight loss might be to much. I'm just looking for other opinions on the subject and should I increase my daily calorie intake even though I feel ok?

Thanks,

Dusty