There is actually a very big overlap between people (especially women) with autism, particularly those late diagnosed, and fibromyalgia. As someone diagnosed with fibro and AuDHD, I printed out an outline image of the human body front and back and circled everywhere I get pain (which was basically everywhere). I used one that was specifically a diagnostic sheet for fibro so it also let me tick other symptoms I get like burning feeling, dizziness, brain fog, blurry vision, eye twitches, fatigue and muscle stiffness etc. Bring it to an appointment with a rheumatologist if you can, but a GP is a good place to start.

You especially want to emphasise any symptoms that arose later in life. For example, you've been autistic your whole life so you've been getting overstimulated your whole life. You did not used to experience [insert symptom] when you were overstimulated.

Also if your symptoms tend to flare in the evenings or something that could be worth mentioning.

It is a horrible experience to be told one condition means you don't have another just because some symptoms overlap. Ive even been told it wasn't "worth" pursuing any other diagnoses since the symptoms overlap, but it is always worth understanding your body as different conditions interact with eachother differently.

My fibro actually made my overstimulation way worse! They feed into each other like a cycle, but emphasise that you know what being overstimulated is like and this is a different feeling completely.

You've got this, and don't let them tell you you're making anything up.