Can't give you a reason why she's doing this, but now that she knows you'll pick her up, rock her and such, you'll have to undue that and teach her to self-sooth herself to sleep. Put her to bed slightly drowsy as before, not sound asleep, leave the nightlight on, and when she wakes and you go in don't turn any other lights on, talk in a hushed matter-of-fact voice, don't pick her up or engage in cuddling, kissing, talking, etc., just tell her it's time to sleep, pat or rub her tummy or back for a minute or so to relax her, say "goodnight" and quietly leave and close the door. You may have to do this a few to many times over each night until she "gets" it, but she will if you're consistent and don't give in to her crying. She really doesn't need to eat after bedtime at 15 months, so just make sure she has milk and maybe a small amount of cereal before bed and don't give her milk during the middle of the night.
Babies and toddlers thrive on routines, so make sure she has a quiet bedtime routine that allows her to relax and prepare for sleep, especially now that shes more active. No TV, loud music or noise, or roughhousing after dinner, just a quiet warm bath, low lights, a story or two and put her to bed.
Hope it all works out soon!