Here is some information about Slim Fast: http://www.slim-fast.com/resources/common_questions/.
They indicate that you can safely eat Slim Fast products while breastfeeding (there is nothing "bad" in them) but to keep in mind your extra calorie needs while nursing. I believe I used the snack bars and shakes while nursing my second child.
I lost all my pregnancy weight and extra weight while breastfeeding my second boy (I'm nursing my third now but he's only a month old). I made sure to eat roughly 1600 or more calories a day (I never count them, just have a general idea and mostly watch portion sizes and food groups instead), ate frequent snacks, worked out for at least 30 minutes a day at least five or six days a week. Honestly, for me, the weight seemed to kind of melt off. I was hoping to get back into my college size (one size down from my pre-baby/post-baby size) but went down four sizes in about six months.
I'm only a month out, but I was getting really impatient myself and have had to be on my feet a lot with the older two kids (4 and 2 1/2) so have had to go sprinting outside after them and pick them up (my toddler is over 30 pounds) and I've had periodic increases in the heavier bleeding because of it. Today I'm having more abdominal cramping (which had stopped completely) because I was cleaning out the basement and moving heavy things. So, try to take it easy for now. I found using dumbbells while seated to be a good way to start toning without hurting anything. Good luck! :)