I barely put any data into my database but aws tells me I’ve exceeded the free tier limit.Why?

Note that there are many resources in the free tier. Maybe you have selected the correct instance for RDS (db.t2.micro, db.t3.micro or db.t4g.micro), but you have also limitations other areas (like storage).

Review cost explorer to check the real usage of all the items, not just the RDS instance.

From memory if you run a resource that's the maximum eligible under free tier for a whole month, when you're 85% of the way through the month this can trigger an "85% used" alert. Doesn't mean you'll exceed the limit that month though unless you really are running more than the free tier allows.