It could be caused by that, although 3 plants in a 55gallon isn't much, and if the lighting it's at least 2wpg, the plants won't soak up much ammonia even if it was more heavily planted.
Sometimes you don't see a high ammonia spike...this tank isn't that heavily stocked. But do keep up the PWC
so you don't kill the fish through nitrite poisoning.
As long as you have some nitrites, the cycle won't really stall.
However when cycling with fish, you always have to slowly stock the tank after the cycle finishes, so it can accomodate the increased bio load.
With fishless cycling, you're adding enough ammonia for a huge bioload, which is why you can almost fully stock the tank once a fishless cycle has completed.