What Are The Chances Of Pregnancy If Experiencing Symptoms After Sex?

Hi im 16 years old im 5'7. And i was just wondering the posibility of be being pregnant. My period is never late, and never early its always been on time. Last month after my period was over I had sex and since then I've been having lower back pains and cramp like pain in my lower stomach, also I've been really sick to my stomach but i never throw up. And that comes with a headache also. And i've also been constipated. And i talked with my sister who had a baby and she said thats how she felt when she was first pregnant and told me to take a test but i wanted to wait to see if i got my period and then it came early and it never has and its always been not really heavy, but never really light either, and its really light and not the normal color either..