Firstly, chance of becoming pregnant on 2nd or 3rd day of the menses is very very remote, for practical purpose nil, however you cannot guarantee anything 100%. Regarding preseminal fluid and ejaculation at the external vaginal opening, one has to be careful. From your qustion, it is clearly evident that there was semen on the vaginal opening suggesting that there was not only preejaculation fluid but also ejaculation. The first part of the ejaculation comes out with a great force. Even when the ejaculation takes place within the vagina, some sperms are known to enter the uterus. So even if you see some semen on the vaginal opening, there is a strong possibility that some part of ejaculate has entered the vagina, which is sufficient for pregnancy. Remember, there are millions of sperms in one ejaculte and to impregnate, only one is sufficient to fertilize the woman s egg. Further more, even preseminal ejaculation also is known to contain few sperms which are known to cause pregnancy. The back ache she is getting is probabally due to the stress and may be the igorous activity if at all she had during love making. Pregnancy does not give low back ache. I repeat again, the chances of becoming pregnant are very very very remote on 2nd or 3rd day of menstruation. The egg is released 14 days before the next expected menses and the life of the sperm is not more than 3 days.