As there are many different Elisa tests being used around the world, it is important to ask at the lab where you tested to find out the accuracy of their test at 11 weeks. They may not know the generation of test being used but they may know the window period.
In British Columbia, Canada, a standard HIV test will pick up an infection in most people between 4 and 6 weeks after they were infected but a small percentage of people can take up to 3 months/12 weeks; It would be very unlikely for someone to test negative at 11 weeks and then positive at 12 weeks.
Unless you had unprotected sex, or shared needles to inject drugs while at the bar in Germany you probably did not get HIV as these are the most common ways infections happen.
It is a good idea to see a doctor to find out what is causing the ulcers in your mouth and throat.
Please leave a comment to let us know if this answers your question or if you need more information.
For other readers, please feel free to leave a comment, or let us know if this was helpful.