If it makes you feel any better, I also had increased endometrial thickness during an ultrasound. The normal range is sometimes listed as up to 14 mm, but sometimes up to 16 mm - that's the number my clinic used. Mine was 17 mm, three or four days before my period was due. I had the ultrasound because I'd been having some bleeding between periods. The 14 mm or 16 mm or whatever measurement you use - if it's thicker, it doesn't mean something is wrong. It's just a guideline. It helps doctors decide whether there is a need to investigate further.
I was sent for a follow up ultrasound the next month by my gp. The measurement was smaller, I can't remember what ... maybe 6 mm, but I was checked much earlier in my cycle. When I saw a gynecologist for the bleeding issues, he was actually annoyed that I'd been sent for a second ultrasound. He said that the 17 mm was totally normal, and that HE wouldn't have bothered doing anything about it and that it had been a waste of time and an unnecessary worry.