Drugs and alcohol use can lead to more risk taking behaviours as judgement is clouded and inhibitions are lowered. This can include risky sexual behaviour potentially leading to exposure to STIs. People who would usually practice safer sex can then be caught off guard and worry about what they may have done.
If you are unsure about any risk you may have taken consider coming to a sexual health clinic for a check-up. If in doubt, check! It is much better to have a test and we will not make any judgements about your lifestyle.