You have had multiple episodes of unprotected sex over the last month or so. Each of these episodes carries a risk of becoming pregnant. Withdrawing before ejaculation is not an effective form of birth control
under any circumstances, because there is usually leaking of some semen prior to full ejaculation.
Therefore, if you have concerns about being pregnant - such as if you are late for your period - then you should see your primary care doctor
or your OB GYN doctor for advice. They can perform a simple urine test for pregnancy and also examine you for any signs of pregnancy.
If these tests are negative, it sounds like you really need to develop a safer contraception plan. Having unprotected sex - even with withdrawal prior to ejaculation - is a very unsafe practice if your goal is to avoid becoming pregnant.
I recommend that you talk with your doctor about this issue. There are multiple contraception options available to you, ranging from condoms to birth control pills. Your doctor will be able to discuss the advantages and disadvantages of all of these methods and help you decide which method is best for you.
Call your doctor to make an appointment at your earliest convenience.