Gracie, personally I have the opposite problem (well more so after DS1 actually) where I couldn't keep weight on while bfing - don't hate me . When I was struggling to put on weight while bfing ( as I was under weight and wanting to TTC), I found out that there are dieticians who specialise in bfing - so they can do up a diet plan to help you lose (or in my case gain!) weight safely while bfing. Perhaps you could try that.

Some other ideas are to do intervals training (google it if you like), where you vary your work out rate over half an hour or an hour so that your heart rate is peaking for just a couple of minutes at a time. Also snacking - eating small portions often rather than large meals, drinking enough water and eating a good breakfast (kick starts your metabolism).

I hope you find something that works.