Tell your GP and psychiatrist as soon as possible I would think as there are a few issues to consider. I know I wasn't given certain meds by my psychiatrist as they can damage the baby if I were to become pregnant.
You need to find out for sure if you are pregnant that will ease some of the anxiety, as you say you think you may be pregnant? Take a test and go to the GP to be sure, then its easier to make informed choices as to what is possible and what you actually want and feel able to deal with, if you are pregnant the gp and pyschiatrist will hopefully support you.
Try to keep calm. Home testing kits these days are more sensitive than those used by the NHS, so you may be fine. Organise a re-test with your GP. They used to test at 2 weeks post your missed period, but this may have changed.
I would take this as an opportunity to discuss the drugs you are using and contraception, but try not to worry too much....home testing kits are very good.
Some meds cause raised prolactin levels. This can stop your periods. You should visit your gp and ask for a blood test. This won't affect your long term fertility, a change of meds will sort it out. Xxx