I think that all kids do different things at different times. I think some are just like that. They learn quick, remember easy, or something and some don't. She may have her own issues due to her being born with drugs in her system. But, I also think that she may just be learning at her own pace, as all kids do at some point. My just turned 3 year old step-daughter, seems so smart, picks up on things fast and amazes me at times. I feel lucky with that. But, I know that another child who is a little younger than her is still learning alot of the things my step-daughter knew at that age. On the other hand, another child I know the same age knows alot more than that one. If that didn't get all jumbled up there, my point is, I think that from you said, she does know a great deal. I don't see it being a problem, but you never know, and I am not a doctor to say what problems she has due to her situation. I guess you don't want any stone unturned, but I don't think you should stress too much on it and make the situation out of hand either.
I know my nieces were so different growing up. I wondered if the one was ever going to talk..she was so quiet. Than all of a sudden she was talking in sentences. Than she had a hard time learning to read, and her mom worked with her on it for a summer and after that- she was fine. She loves to read, it's still a habit for her to read with her mom at night before bed. Because she wants to now!
I think you should watch out for things due to the situation, but also not stress about it all too much!!!