RDS still 18G available but exceeded 85% of the usage limit for one or more AWS Free Tier-eligible services for the month

It's based on the amount of storage allocated, not the amount that's used. So if you've allocated 20GB but your database is small and only has 1GB of data in it, it's still the 20GB figure that is used.

The good news for you is that Free Tier includes 20GB of storage for RDS per month https://aws.amazon.com/rds/free/ so you shouldn't be charged for this.

The way this is calculated is basically you can allocate 20GB at the start of the month, and keep it for the whole month, and that counts as the full 20GB/month (you could also allocate 40GB and keep it for half the month, or 60GB for a third of the month and the arithmetic would still work, but that's not relevant here).

It's likely that you got the "85% warning" when it was 85% of the way through the month where you have allocated the full 20GB.

Go to https://console.aws.amazon.com/billing/home#/freetier and it will show what your Free Tier entitlements are, and how much you have used of each so far this month, and what the projection is for your usage by month end.