My girlfriend and I had sex 20 days ago for three days, but I never left my sperm inside her. But it's been one month from her last period, and she has not had a period yet. She feels she might be pregnant. So please tell me she won't get pregnant and some ways you can avoid pregnancy after sex.
The only way to avoid pregnancy is to not have sex. Every act of sex contains a risk of pregnancy, regardless of the precautions taken. To stop a pregnancy after it has started is to kill the child you have conceived, and you can't fix a sin with another sin.
The mistake you made was thinking you could avoid pregnancy by not ejaculating in her. A man drips pre-ejaculate fluid when he is aroused. That fluid can contain sperm. Thus, you have been putting sperm into your girlfriend each time you put your penis in her. While pre-ejaculate fluid doesn't contain a lot of sperm, it only takes one reaching the egg at the right time to cause the egg to be fertilized.