Unusually high AWS monthly bill due to Directory Services

Hi there, I had a unusually high monthly bill, due to "Directory Services". This is a service that was setup in August 2022, due to some tests I was running, but, for some reason, last month, the costs run up to $38. I deleted the Directory Services in the Oregon region, but can you please help me understand why I was billed that high and how to avoid it, going forward? Thanks. Fotis