You didn't really say how big she is. But I'll tell you that my 10 month old daughter is almost 30 inches tall, weighs 24.5 pounds, and is somewhere in the 90's for percentile in both height and weight. When she was born, she was somewhere in the 5th percentile...not early, just a teeny baby (17 inches, 6 lbs 15 oz, but when I took her home she was like 5 lbs 4 oz).
So now, I think she is just a huge baby...she seems so tall, when she stands up and holds onto the kitchen table, or I feel like my back is breaking holding her...but she isn't chubby really at all. She is still exclusive breastfed (she eats food, of course, but no bottles or formula.)
However, I don't think you have anything to worry about that she's dropped to a mid percentile. What you have to remember is that ANYTHING between the 5th and 95th is considered normal, that's just the bottom and top of the "normal" range...it's falling OUTSIDE of that range that isn't considered normal. As long as they move together (height and weight, and head circumference) and she is healthy, you have nothing to worry about!
Be grateful you don't have a huge baby! :)