A cycle as short as 18 days once in a while can happen from stress, travel, or a one-off hormonal shift. If it repeats for two or three cycles in a row, it's worth getting checked.
Next Steps
Track your next couple of cycles and see a gynaecologist for a hormone panel if the short gap continues.
Health Tips
Fertility is generally highest through your twenties and early thirties and declines gradually after that. Since you're already thinking about this, a preconception checkup now is a reasonable step.