You haven't got cancer you are just going through a very shitty process until you get a diagnosis.
You need a specialist so push for a referral to a specialist centre. There are some great drs out there and you will find one. I've had similar issues, I got to the point I saw 7 drs in about 6 months until I found the right one, this wasn't in the UK but in a major city of what was supposed to be medical excellence. One of them prescribed me so many painkillers that the pharmacist was concerned!
The celiac thing is interesting. I got tested and it came up with mild intolerance for wheat and a load of other stuff. But not celiac. I think it's because endo is hormonal and hormones can cause changes in your digestion plus if you have endo near your bowel it can cause problems. (And if you look at how it's all arranged in there we have loads of organs squashed in very close together.)
My advice now is you need to collect yourself difficult as it is. You will want to talk about how upset you are feeling etc. But you will only get 20/30 mins appointment time. Write a list of symptoms and a short list of questions you want answered.Take notes/if you are likely to get upset, take friend/partner or parent.
They can't diagnose you on the spot, they will send you for an ultrasound or MRI, this won't necessarily show much up. You have to push for a laprascopic surgery. You don't want too many surgeries because it creates scar tissue try and get a surgery where they cut it out not laser as it isn't as effective not just a surgery where they diagnose. You need a good surgeon. A good surgeon will believe you and give you solutions. I've only met one so far - Google it.
Short-term get your periods stopped I took Yasmin for years and tri-cycled so only had four periods a year, visanne works for endo but not in UK yet. They also have prostap injections, these can cause hot flushes etc but shut it all down for a while.
I'm really sorry you are having a crap time with it. So many woman have dealt with this misdiagnosed. Lena Dunham and countless others have written blogs etc about it. Reading as much as you can on here can help. You aren't alone and you will feel better soon xx