Thank you for consulting with Doctor spring. I understand that you are worried about the possibility of a pregnancy and that right now it's not something that you're ready for.
I'd like to answer your question with a brief description of the normal menstrual cycle. The Normal Menstrual cycle has a duration of 28 days. But even normally, this can vary between 20-36 days. There are several events taking place in the body that vary depending on the halves of the cycle. The 28 day menstrual cycle can be divided into 2 halves. In the first 14 days, the uterus makes itself ready to receive an embryo if the ovum is fertilized. So, the lining of the uterus called the endometrium builds up, the ovary prepares itself to release the ovum, the cervical mucous is thinner in the first 14 days and the predominant hormone is estrogen. The ovum is released on the 14th day of the cycle. Usually the date of ovulation falls on the total duration of your cycle minus 14 days. For example, if you have a 34 day cycle, it is most likely that your ovum will be released on the 20th day. So you have more chances of getting pregnant if you have sexual intercourse 2 days before or after the 20th day giving adequate value to the life span of the sperm and the ovum.
So this means that if you want to avoid a pregnancy, one of the natural methods of contraception is the safe period method in which the user avoids sexual intercourse in the days she is likely to be ovulating. In your case, you have had intercourse towards the end of your cycle when you're going to start menstruating but you haven't got your periods yet. So, based on the above explanation, it is quite unlikely for you to be pregnant now. But, there's always a chance that you ovulation may have been delayed. This could be the case especially if you have irregular cycles. Another thing is that you mentioned having done a UPT on yourself which turned out to be negative. Urine Pregnancy tests are very sensitive if you are pregnant. But the important thing is that they need a minimum amount of hCG (which is the hormone produced in pregnancy) to become positive and that is reached only towards the late 2nd week into your pregnancy (if at all you are pregnant). So, a UPT will not be positive now even if you repeat it again and again.
The pain that you mentioned in your lower abdomen could be due to Pelvic inflammatory disease or it may be due to your anxiety itself. Presently, there is no way of telling whether you are pregnant in less than a week. But what you could do is wait 2 days more for your period and suggest that you relax and visit your Doctor who will be able to give you more information on this and more contraceptive options.
Hope this information was of use to you.