two negative test but one positive

i have been married for 5yrs but had been having sex with my husband a year previus to our marriage. On Dec. 00, i got tested for chlamydia because i got pregnant and it came back negative (By this time, we had been having sex for a whole year). on my 6-week postpartum, Sep. 01, i got re-tested again and it also came back negative.  about a year later, on Feb. 03, i got tested again for chlamydia and this time it came back positive. both myself and husband got treated and now we are both negative. i have always had a vaginal test done.  
I am very confused and about to get a divorse. My husband swears he did not have to do anything with anyone else. I was aware that he had a previous partner before me and he sais he got it from that previous partner. it seems to me that my husband had something to do with someone else after i had my baby.  
is it posible to have two negative test in a row for chlamydia and all of a sudden a positive one?
If this is true, what is the percentage of that?
based on my test results (2 negative and 1 positive after 4 years of having sex)can i assume he cheated on me after i had my baby?
i am not sure if i can rely on his story. i had never showed any symptoms of any kind ever and my baby is fine.
Please help me, i feel very confuse

[H. Hunter Handsfield, M.D.]

It is very unlikely you carried chlamydia for 4 years with 2 false-negative results, followed by a positive result.  Therefore, either your husband (or you) had sex with someone else, or your 2003 test result was falsely positive.

All chlamydia tests sometimes give false-positive results, although some tests are more likely to do so than others.  Only you can judge your husband's truthfulness about other partners.  If you have no other reason to distrust him and he seems to be honest in denying other partners between 2001 and 2003, you probably should believe him, not the test result.  In other words, the appearnce of chlamydia in these circumstances is a clue that you or your husband had other partners, but not definite evidence; that is, you cannot "assume he cheated", as you put it.  You'll need to trust your judgement of him and what he is saying.

Good luck--  HHH, MD