RDS Storage Size Full but actual DB size very low

Hello, Our RDS storage size was 100GB. Actual DB size was 18 GB. But suddenly storage get full & after some time it was automatically decrease. We have checked DB size at & binlog size, both are normal. Kindly let us know possible reasons to suddenly increase RDS storage usage & auto-decrease size also. We are using RDS Mysql 5.7 engine.