I have had sexual health check and I've just been told I used to have chlamydia but am now clear. I do not know how this is possible?

If you were treated for some other problem with drugs that may have also cured it. About the only way I can see it being possible

Chlamydia-Treatment Overview (http://www.webmd.com/sexual-conditions/tc/chlamydia-treatment-overview)

I may recommend a closer check by your doctor