Symptoms of sexually transmitted infections (STI's) are NOT always noticeable. It is important to be regularly screened for STI's as they can cause severe lasting damage if not treated. It is recommended that young people that are sexually active are screened every 6 months.
For information on symptoms to look out for, what STI's can do to your body and where to get tested go to this handy NHS symptoms checker. https://www.nhs.uk/conditions/sexually-transmitted-infections-stis/