Im 18 and my periods are always on time . So could I be pregnant??

Not likely. There are many causes for periods to be late, anything from medicines to changes in your body to changes in your physical activity. The flow will vary from period to period. Its rare for your period to be exactly the same every month. Few women have it hit the same day every month and to be the same flow or last the same length of time every month. You will have to learn what is within the normal ranges for you. Little things will cause minor changes with you. If you are sexually active, you should always take responsibility for your own birth control no matter what your partner claims to be doing about it. It's always better to be safe than sorry. Leaving it up to someone else isn't smart.