Leopard gecko skin gone darker + looks tired all the time.?

I bought a female sub adult super hypo tangerine carrot tail gecko 3 days a go. I bought it second hand and she was really tame. It was kept with it's mother and another gecko (both female) however when i bought them back to mine, i put it on it's own with a heat mat, whilst the other 2 arein the breeding tank (males not in there yet, im giving them time to settle down). The 2nd day the baby's skin start going darker and now (on the 3rd day) it's quite dark. Orinially it was bright yellow and now it's gone to a darker yellow. Also, the skin between her 'bumps' on her body, some of it looks sandy?and she's kept on aspen bedding as a substrate. Do i need a bulb to brighten her skin? or should i just leave her? any advice would be amazing. Please help. Thank you.