Hi Nae
The cervix does naturally shorten as a preg approaches FT & 3.5cm is still a great measurement so try not to worry too much if you can! You are having pretty regular scans arent you so thats excellent. 2.5cm was the benchmark I was aware of in my case too although I had the stitch placed at 14 wks.
My cervix halved in length between 19 & 21 wks & so I had hospital bedrest for 3 weeks which brought my length back up & continued rest kept it stable & I made it to 38 wks when the stitch came out. I have heard that 18-24wks is the highest risk period for IC as this is when the baby is starting to put pressure on the cervix. It certainly seemed to be the case for me. Although I know they dont think you have IC the same pressure theory would apply I guess & this might help explain the fluctuation.
Anyway, sorry if none of that helps - hope it makes some sense
Good luck!!