Is this the sign of what's to come?

My 2yo filly has always been extremely quiet, she's been on our yard for a year now and has never kicked, bit, bucked, bolted or reared. However yesterday as I was walking her back towards the field she bolted and was cantering circles round me and bucking. Plus in the stable a week back she had been in over night and started rearing the following day, she has been fine since but hasn't been kept in over night. Could she be feeling a bit better with regular feeds and could this be what she's going to always be like or could this be her just being a big baby?