Hi Lucy,
don't think you are overreacting, you know your dd best, and you will spot anything unusual.
film it on your phone if you can, that's an excellent idea, it's practically impossible to describe these episodes to the paeds properly.
I really hope its nothing to worry about, but it does sound similar to my son, and he had absences ( momentary vacancies) when he was little, also little startle jerks like babies have but he still has them and he's five. These absences and startles, were a both kind of epileptic seizures, but they didn't seem to bother my son, and were only momentary. The sharp cry and then go rigid might also be a kind of seizure, do you think? Have a look on the epilepsy action website, they describe all the different kinds of seizures, and what to do.
DS had an EEG about a year ago, and they found significant unusual brain activity, (epilepsy) And then, after Christmas this year he had some big seizures. All controlled by medication now, thank goodness.
Defintitely keep a diary, it's so easy to forget. And do mention it at your appointment. If you are worried about dd's development, ask the doctor if they can refer you for extra help, eg speach therapy, physio etc. Or if dd is very young, portage were brilliant for my son.
Don't panic, but do get informed so you know what to do just incase she has a fit x