I'm no expert but from my experience if she has a day nap everday (routine) she should sleep better at night. If she misses her day sleep she will be over tired come bedtime & behave the way you've been describing, well if my DD missed her day sleep at that age, I would have probs getting her to sleep that night. I still had her in a cot at that age so I could put her in there & she just had to stay in bed whether she wanted to or not until she was too tired to fight it anymore & fell asleep.

You say its since she started FDC, do they give her a day sleep? If not, that may be the cause of the problem??

IMO At 20 months old she would still NEED her day sleep

Someone else may be of more help, as I said, this is just from my experience