Firstly, having an early period does not necessarily mean that you are pregnant. There are plenty of reasons for why this may be occurring.
If you are on birth control pills, these tend to result in early periods or bleeding outside your regular times.
You could also be bleeding early due to:
The name given to the imbalance of hormones that causes irregular or early periods is ‘anovulation’. This term refers to the imbalance of hormones from the hypothalamus, ovaries and pituitary gland. Symptoms are missed periods, bleeding between periods, heavy periods, and periods that last longer than seven days.
The best thing to do, if you think you may have this condition, is to keep a record or calendar of the dates you bleed, for at least three months.
If your period occurs shortly after having sex, this again does not necessarily mean that you are pregnant. Emergency birth control - such as the morning-after pill - can sometimes result in bleeding, and if it is just spotting, then this is not unusual, particularly after your first time. However, if you are experiencing pain when bleeding, you should get yourself checked out by a doctor.
Most people would assume that if you are having periods, then you are not pregnant - although some pregnancies can involve periods throughout, though this is very rare.
If you bleed often and irregularly, you should seek medical advice.