Why does the fat turn solid during fat transfer over time?

Hello Angelinelovelace,
The consistency of fat is different in different parts of the body. Also, during the liposuction portion of a fat transfer operation, the first step is to inject a large volume of "tumescent" solution, which is saline mixed with epinephrine, likely with anesthetic since it sounds like this was an awake procedure. 
At the beginning of the operation, the tumescent is still present to some extent in the areas that are being aspirated; the fat that is "sucked out" therefore is more watery than it is at the end when most of the tumescent has either spread throughout your body, been "sucked" up or has drained.
Additionally, during processing of the fat, the aspirated fat mixture is typically purified via centrifuge, dehydration or a variety of other techniques to achieve a toothpaste-like, thick consistency. 
In summary, there are a variety of reasons why the fat could have been thicker at the end of your operation - but in general, plastic surgeons do want the fat to "thicken" before injection!