Hi jackie,
That is frustrating because it sounds like you did everything right and the test seemed to go smoothly. That does make it harder to get to the bottom of exactly what happened.
My guess (and this is just a guess based on what you've written) is that it was likely due to anxiety/stress, but that you just weren't aware of the effect that it was having on you. As an example, sometimes when I'm reviewing a practice test with a student, the student will tell me that they have no idea why they chose a particular answer, that they have absolutely no recollection of choosing it. I expect that this phenomenon is even greater on the actual LSAT than on practice tests.
As for advice, I'd focus on the emotional/mental side of taking the test. There are various activities (meditation, visualization, etc.) that can help get your mind in the zone, so to speak. Athletes often use these techniques before a big competition.
Here's a link to some resources on test mentality for the LSAT that you may find helpful.
https://blog.powerscore.com/lsat/the-ul ... urce-list/
Other than that, try to stay healthy, get plenty of rest, and try to relax and get your mind off of the LSAT for the next few days.
Good luck!