Hi there.. it's always safe to use protection or take post coital pills(which can be taken upto 3-5days after intercourse) if it was unplanned... if you sure you did not enter her nor did you ejaculate into her then nothing to worry, her symptoms could be just due to gastritis.. but if you have entered, ejaculated then chances that she can conceive are high as you have had intercourse during her mid cycle which is most unsafe time as she wil ovulate mid cycle and chances of fertilisation ie chances of pregnancy is high.. now ask her to take medications for gastritis like pantoprazole40mg , wait and watch for her symptoms to settle.. then after 21st May if she doesn't get periods then you can get urine pregnancy test done and you know whether are not she is pregnant... if she gets periods on time then well no worries but be sure you use protection always in future..