how far along am I ?

I did blood test's 23 weeks ago and the blood tests came back possitive the tests shows that I am 4 weeks pregnant. I cant remember my period all I know is that it was in July 2008. the sonar scan show's I am 23 weeks pregnant this week. I am worried because me and my Fiance was seperated and only got back together on the 25th of July I was with someone else on the 11 and 18th of July. is there a possiblity that this child is not my Fiances??

my due date is 14 May 2009 according to the doctor????
please help I am so worried??

thank you
Nienkie