Well what you read about the chemo only effects a few people anyway so its still rare they test positive at a later date, the part you missed is that they will be on treatment before and currently whilst testing. AS this is not the case thats somthing you dont have to worry about.
As joggen said, you 8 week test is very near conclusive so take your 12 week test as routine. The fact you didnt get any std's would indicate only good news.
Overall you had a risk but you look like your going to be fine, but in the future use condoms, and if they dont want to rubber up then you dont have sex