Why can't I get Pregnant?

I am 28 will be 29 in July. My goal was to have four beautiful children by the time I was 30. Well it doesn't look like that will happen. I have two beautiful children now. A 4 year old and a 2year old. One boy and one girl, I got lucky. However for the last year to year an a half we have been trying for another. I had a check up to make sure everything was fine. They ran some blood tests on me and I am great and healthy. I should mention that last year I was pregnant than had a miscarriage, I was about a month along. Ever since that miscarriage my body has been screwed up. Everytime I think I am pregnant my period comes knocking. I have recently had my period and I had taken a test a day before it showed. I could have sworn I was pregnant. I have gained some weight in my stomach, I have had sore breast, and I have been feeling sick. Then after getting a negative test and my period I thought nothing of it. However, I am still gaining weight in my stomach, my breast are still sore from time to time, and I still feel sick. I have tried contributing it too food but when someone else eats the same thing as me and doesn't feel the same way it is hard to do that. I try to think of everything it could be with pregnancy being my last thought because I don't want to get my hopes up. My husband is 31 and is about done with kids but I would still like to have one more but for some reason we can't. We had no problems what so ever getting pregnant with our first ones. The only thing that has ever happened to me has been the miscarriage, nothing else has changed in either my husband's life or mine. Please help anyone. Why can't we get pregnant now?