I'm not a mum yet obv but my mum was a childminder for 13 years up until a few years ago and she looked after one little girl who's behaviour was a lot like the behaviour you're describing and in the end the doctor said it's because she's too advanced for her age. From being a baby she used to cry constantly because she wanted to be doing things. For example when she was about 9/10 months old it was near impossible to get her to sit anywhere because she wanted to be walking when she couldn't - it was as if she wanted to be independant and do things for herself and got really frustrated when she couldn't and would often lash out as she got older as a result because she wanted to find things more challenging - as Chloe said.
Now she's in school and going to year 1 after summer and already she is top of her class and they were thinking of moving her up into the next year because of how able she is for her age.
Hope you get sorted (: x