Im unsure when to do a pregnancy test ive had unprotected sex week before period which started on the 18 an only lasted 3 days and up after?

It's generally recommended to wait at least one week after your missed period to take a pregnancy test. If you take the test too early, the level of pregnancy hormone (hCG) in your urine may not be high enough to be detected, and you may get a false negative result.

For some tests, you may need to wait longer than a week after your missed period. The instructions that come with your test will tell you how long you should wait.

If you're concerned that you might be pregnant, it's a good idea to take a pregnancy test as soon as possible. This will allow you to get the care and support you need if you are pregnant.

Some reasons why your period may have been short include:

* Stress: Stress can affect your menstrual cycle, causing it to be shorter or longer than usual.

* Diet: A drastic change in diet, such as starting a new diet or losing a lot of weight, can affect your menstrual cycle.

* Exercise: Intense exercise can affect your menstrual cycle, causing it to be shorter or longer than usual.

* PCOS: Polycystic ovary syndrome (PCOS) is a hormonal disorder that affects the menstrual cycle. PCOS can cause irregular periods, including short periods.

* Menopause: If you're over the age of 40, you may be experiencing perimenopause, which is the transition to menopause. Perimenopause can cause a variety of menstrual changes, including short periods.

If you're not sure why your period was short, it's a good idea to talk to your doctor. They can help you determine the cause and recommend treatment if necessary.