Dark colouring on the belly doesn't necessarily mean impaction. Some internal organs are darker so are more visible.
What else makes you think it's impaction? If you're concerned that this is the problem the best thing is to take her to the vets. If impaction is left, it only gets worse.
More details will help. Explain the setup i.e. temperatures, what heat sources you have, if they are statted, how you are measuring the temperatures, substrate, any lighting, food items, supplementation etc.
A picture of the gecko will also help. Do you know what morph she is? Enigma morphs exhibit this kind of symptom (as well as circling, rolling, bad aim etc), it's a neurological problem that cannot be cured, however symptoms can come and go.