Q:

AWS - False Billing Alert at 0.00 USD - at start of the Month

I have a billing Alarm set as Whenever charges for: EstimatedCharges is: >=USD $100
So i assume - The alarm should trigger when my Billing cost is above 100USD.
But early today i had a Billing Alert for my Alarm
That said

The alarm limit you set was $ 100.00 USD. Your total estimated charges accrued for this billing period are currently $ .00 USD as of
  Saturday 01 December, 2018 11:24:23 UTC

But when I checked CloudWatch the alarm state is OK

State changed to OK at 2018/12/01. Reason: Threshold Crossed: 1 out of
  the last 1 datapoints [0.0 (01/12/18 05:24:00)] was not greater than
  or equal to the threshold (100.0) (minimum 1 datapoint for ALARM -> OK
  transition).

A:

For each alarm, you can set up 3 types of alert depending on the alarm state (OK, ALARM, INSUFFICIANT_DATA).
When you first create an alarm, its initial state is INSUFFICIANT_DATA. Then, depending on the metric value, threshold and period, it will reach the OK or ALARM state. 
If you have set up notifications for all the different states, it's normal that you get a notification as well when the alarm went back from ALARM (end of a month over 100$) to OK state (new month - new bill). 
If you don't want to get notifications when billing is under 100$, just remove the OK notification that you have set up:

Hope it helps!

