Help, who is the daddy?

Hi I have a major issue! I have ben ually active since I was 18 and only slept with 4 men. I am 26years old now. Most of the time I would not use protection . I have been with a man for 5 years and although I have never used protection i never became pregnant. I have never had kids because I could not get pregnant. Well, about five months ago I cheated on my boyfriend, in fact we separated becuase he found out about it. Although being with him I never got pregnant, two and a half months ago I did. The guy I cheated with says he had a vasectomy done 10 years ago. I am really confused because for five years I did not get pregnant from my boyfriend and less than three months after I started sleeping with this other guy I did. Help, I do not know who the daddy may be. Is there a possible way the vasectomy could reverse on its own?