This is my uneducated guess.
You've psyched yourslef into believing you are slower in the fist 50m due to your looking at your times and realizing there is a problem. Now you are trying too hard over the first 50 in the 100, straining tensing, etc, which is causing you to accelerate slower than if you were relaxed. I think thats why your 100 time is slow compared to the 200 and 400, because you are more relaxed in a longer race due to the fact that the races dont rely on a great start as much.
I hope that made sense, but thats my only guess.