when i checked my Ir it was 1.23 so i played a game of inferno i had 27 frags 18 deaths! i got ranked up from gold serpent to bronze badger but my ir decreased from 1.23 to 1.19 and my stats also decreased! please check into this. https://www.sostronk.com/pug/5285320207892480 here is the inferno match!
Profile stats are calculated only for the last 30 days, so what you described can happen.