About 1.5 million women in the UK are thought to have the condition, which causes pain and extreme tiredness as a result of tissue similar to the womb lining growing elsewhere in the body.
When she was 13 years old, Ms Thomas said she remembered "keeling over in agony" at the back of the classroom.
"I knew something wasn't quite right," Ms Thomas said.
"My skin seemed a lot worse than anyone I knew, my back pain was crazy. It didn't seem quite normal to me.
"Three weeks out of every month I was going through that."
Her doctor put her on the Pill to help manage the pain but it gave her migraines, so she came off it in 2018.
The pain then grew worse, and her doctor said it must be caused by "anxiety or over-exercise".
It was only after her sister googled the symptoms that Ms Thomas was able to return to her surgery and ask to be tested for endometriosis.
The only treatment offered to her was a Mirena coil, and she now feels she has been "put on the back burner" with no further medication.
She has now found a support group where people with the condition can talk about their experiences.
"It's good to speak to other women who have it," Ms Thomas said.
"There is no cure. Pain management can be difficult.
"I mask it a lot at work and in relationships because you don't want to be a burden.
"There needs to be more research and funding. And making more space for women."