Don't know if it wil be my chid

Don't know where else to post my question but i have just recently found out a girl i am seeing is pregnant.  she did not tell me till today that she was fooling around with her ex around late sept. early oct.  The fist time we had started having sex was around oct. 20th.  found out on the 30th of nov. that she took a pregnancy test and it came back positive.  I am worried it may be mine but i do not want to get attached and find out later on when the baby is born that it is not mine.  Is there anyway that i could found out when the baby may have been concieved or possible dna test?  My head is spinning bc we do not love each other nor do i have the money to raise a chid.  To make matters worse she has 2 children from her ex.  Please any suggestions woud help me greatly.