1st. That thing she does with the startle is called the startling reflex. It is normal. My son did it alot, but my 6 month old doesnt do it much.
2nd. As long as her sleeping time totals about 12-14 hours she is ok. My daughter, the 12yo, stopped taking naps totally by 9 months. She slept 12 hours a night. Every baby is different.it doesnt matter if it is uninterupted sleep, just add up her sleep time for a few days. My son woke up every 4 hours until he was about 6 months old, then every 6-7 hours until he was about 1 1/2. My 2 daughters slept through the night by about 2 months old.
lastly, about the nightmare thing- It is also normal. A lot of babies wake up sometimes screeming. It could be gas pains, or something.
I would mention it to the dr. too. I had to get a doctors note for my daughter when she started going to preschool because they expected her to lie there in the dark during nap time for 2 hours. She got in lots of trouble because a 3 year old can not do that. They finally let her go play with the older kids during nap time.
sorry this is so "messy" I am typing one handed. (popcicle):)
good luck, i am sure she is fine. If she is happy and growing then she is probobly ok.