What should i do ?

My ex boyfriend cheated on me i found out when i went to the dr for a check up & they told me that i had chlamydia i told him what they said and that he should go get a check up as well so he could also be treated for it but he refused & i ended the relationship now he has another girl friend and I want to tell her but i don't want to come off as "that ex" how could i tell her without seeming that way ? I honestly just what to warn her and not seem bitter but she's very rude she's messaged me before about random things when me and him do not talk at all.