Need advice

Okay so my normal weight is 135. I had 2 babies back to back within the past 2 years so I haven't been my normal weight in about 2 1/2 years. With both I got up to 200 lbs. anyways 6 weeks ago i had my daughter and immediately after her I was 190 so since then I've gotten down to 170. I've been dieting and not cheating and this past week started working out 45 minutes for 6 days. I thought for sure I would lose more weight. Instead i gained a pound. I'm so disappointed and frustrated. I also should mention I'm breastfeeding exclusively and I've been keeping my calories between 1,000 and 1500 a day eating things such as salad,nuts, chicken and vegetables. So I'm not sure what's going on. So for those that have lost large amount of weight have you been through this? Is it normal? I don't want to keep working out and eating healthy for nothing. My goal is to lose 40 lbs within the next 5 months or so. Being realistic