I was positive he was negative and he cheated!

Here is the situation.

I started dating Jon in July of 2011.  We slept together unprotected until January 2012.  in this time he was sleeping around with his co-worker and was unprotected.  i found everything and got std tested!  My results came back positive for chylmidia (unsure how it's spelt)  i took the antibiotics and told him and his co-worker to get tested...  well there results came back negative!  how could this happen?  i was faithful and he wasn't however i was tested positive.  I had an STI test before i was with him and only have slept with him after that initial test that was negative as well.  Is there anyone that can gie me some insight into this??  Can he not test positive but have carried it to me?  Could my other infection (not sti) caused the false reading?  i'm just so lost and confused and i don't know how i can have it and he doesn't...