No one can just know you are pregnant if you had unprotected sex. You will have to take a test. But . . . ALL women/girls can be late from time to time. Things that make you late are illness, stress, anxiety, changes in routine, weight gain or loss or just a hormonal shift. However, if you aren't tracking your cycle so we don't know when you may be in a more fertile time of your cycle, there really isn't much to do but test. Day one of your period is cycle day one. Women are usually more fertile around day 9 to 15 for the average cycle and sperm can live for up to a week inside of you. Ovulation occurs for 24 to 48 hours every cycle. However, there is no SAFE time to have unprotected sex.
So, if you are late, go buy a pregnancy test and take it. And if you are not pregnant, then please look into getting yourself on birth control pills or buy condoms so that you don't wind up with an unplanned pregnancy. good luck!