The livebearer gravid spot is one of the most commonly misunderstood aspects of livebearer reproduction. If you try to judge pregnancy and such from gravid spots you will not have a lot of success with these fish. I can tell when my guppies are a day from giving birth with almost 100% accuracy without even looking at their gravid spots.
A gravid spot develops in juvenile females whether or not they've been fertilized yet. This dark patch is actually a mass of orange eggs but due to the colouration of the guppy often appears black or brownish. The gravid spot changing colour therefore, is a result of a change in colour on the guppy's part not anything going on with her offspring. If a fish changes colour and it isnt breeding time, chances are you've stressed it. You said you'd had this guppy a day before tossing it in a net and if that's the case 100% you've over stressed that fish. A new tank is stress enough for a fish without being tossed into a tiny net right away.
Size isnt neccessarily an indicator of pregnancy either. Shape is more important than a female's overall size. Sometimes a fish is just fat because it has eaten a lot and when you stress it all that was inside comes out. Once they get back to eating normally the weight usually comes back. Absorbing fry is a very uncommon occurence, far more so than most would lead you to believe. Unfortunately, if this does occur your fish will not just carry on normally in all likelihood. A fish that has absorbed fry often has serious problems later, including an inability to reproduce.