PT100 is 0,4 Ohm/K. Or 0.0000000004Ohm/nK. This means, at 100Ohm, you are measuring 4 parts per (american) trillion for that nK.
Good Luck and please, report how many of those digits were pure noise!
Near linear but usually even for industrial use there is a three term polynomial correction applied.
There's a whole standard devoted to the subject, DIN/IEC 60751.
Useful reference from TI : https://www.google.com/url?sa=t&source=web&rct=j&url=https:/...
PT100 is 0,4 Ohm/K. Or 0.0000000004Ohm/nK. This means, at 100Ohm, you are measuring 4 parts per (american) trillion for that nK.
Good Luck and please, report how many of those digits were pure noise!