It seems like a possibility as being thin isn't really compatible with being pregnant in my experience!
I think what is more likely though, is that you are not 'letting go'. You probably want a baby so badly that you're thinking about it a lot and trying to do everything that you can to make that happen. You may just be trying to hard to force the outcome. In terms of letting go, have you ever manifested anything else successfully? If so, try and use a similar process here.