Shes been cheating ... Is the baby mine?

I am sympathetic to your plight especially now that a baby might be involved. I see three issues here that you have to deal with. Let us begin with the one you asked and which is the most important. 1) Based on the dates you provided, there is a 50-50 % chance that the baby is yours. According to the information ,you slept with her on the 26th and her period ended on the 21st. Though close ,you could be the father.Unless the information you have is not accurate or she lied to you about the mystery guy and when she slept with him or if she slept with other guys or was already pregnant when you slept with her on the 26th,you could be the father. Only a paternity test known also as a DNA test could definitely and difinitively answer the question of who her father is after she is born. 2)That raises the question of whether you should continue to care for her during her pregnancy. This is a dicey one.If you refuse her help and the baby turns out to be yours,then the guilt and ramifications might be too much for you to deal with .On the other hand if you take care of her and the baby turns out not to be yours then you might feel like a chum. The only possible alternative ( and I am not sure if you would like that) is to have a legally binding agreement. First you would ask her if she is positive that the baby is yours and not someone else including any other guy you might not know about.If she says yes,then you would draw up an agreement to be notarized which would say something like" if DNA paternity tests confirm that the baby is yours ,she would owe you nothing. However,if the tests show that she lied and the baby is another man's baby,then she would have to repay you every penny you spent caring for her;she can collect from the actual father if she so desires".During her pregnancy you guys would keep a running tab of the expenses and the receipts would be signed by the two of you so there would be no future arguments about them. 3) This is the future state of any relationship .You did not say your current feelings for her(which is different from your justifiable anger and sense of betrayal) but it is tied in to number 1 and 2 above.You have to resolve that point whether you would be willing to forgive her and stay in the relatioship if that proves feasible.If it is, then you might want to reconsider all of what I have said. I wish you all the best.