I had a similar experience last night. Took at 9pm, read for an hour and then was awake at 12.30 with my heart beating fast and sweating like a pig and felt sick.
Couldn't get back to sleep for ages.
Usually I get a couple of hours sleep and then toss and turn all night but I do go straight back to sleep. Last night I just lay awake for ages after being woken as I felt alert?? Odd.