What do I do?

Hi guys I’m currently off work with sever pain in my abdomen, they are doing tests and are investigating whether it’s endometriosis. Which is great because I’ve been dealing with this pain for years. The only problem is work. They don’t believe me. I don’t know what I can do I can’t go into work like this because I won’t be able to concentrate or tbh even get there (getting up is an issue when it bad) thing is I experience pain most days and still go in I only take time off when it’s bad but my work seems to think I take time off for small issues because I don’t have an official diagnosis. But that doesn’t mean that it’s not there the doctors just haven’t figured it out yet. Help. I don’t know what to do. I dont want to lose my job.