Abrupt Breaking?

Last night my mechanic was testing out my car to see why it wasn’t running efficiently. Turns out something in the engine was disconnected and he reconnected it. We took it for a test drive on some back roads and as I was trying to get my seatbelt on he just starts speeding fast and then abruptly stopping. I was completely unprepared for him to start speeding as soon as we left the parking lot because these roads weren’t spacious at all. I was putting on the seat belt as we were leaving the lot and after the first speed up and stop I was still struggling to get it on and lock the belt down and by times 2 and 3 it was already on. My question is would these abrupt stops be enough to cause a concussion? This must sound ridiculous but I’m mainly worried about the forces causing some kind of whiplash or concussion. Does this warrant a doctor’s visit or scans? I did not hit my head but I could feel my body slightly moving up every time he braked. The whole thing just has me shaken up because someone tried to exit their neighborhood too and almost hit us because of how fast my mechanic was pushing the car on the back roads. Any advice appreciated as I’m trying to be more self aware and proactive with my health by taking these things more seriously.