Hi, I Just Learned My Partner Has Clymidia And Have

Hi, I just learned my partner has clymidia and have several questions:
When going for a regular check up and getting blood drawn, is it customary to check for STDs?
How long can Clymidia go undetected in women? Years?
If neither partners had Clymidia prior to intercourse what could any other explications be?

Thanks