Losing the BabyFat

I have been on a diet forever. Sometimes I had success while other times I only lost a little at the start of the diet. I had sworn I couldn't lose weight anymore at 35. Then I became pregnant. I was happy about having a baby but worried I might never lose the weight. I gained 35 pounds. That's not really the end of the world but my Obstetrician would have liked that number to be less.
I actually lost the majority of weight though the first 2 and a 1/2 weeks. It took a few more weeks to lose the last nine pounds of baby fat. I had very little trouble. I ate about 1,800-2,100 calories so I was not starving myself. I exercised a little bit but I really could not do anything strenuous the first six weeks. It was also winter so I couldn't do much walking. I tried breast feeding but because of complications I had to pump and supplement with formula. I am on Thyroid medication but I have already been on it for years. So how did I lose so much so fast? It may have been the combination.
Now, the funny thing is I'm back to my old weight and feel like I can't lose a pound again. Of course it's all in my head but it's hard to get past that mental block. Some have said I looked like I lost but I don't really think it was that much.