hyper 8 heat problem

HI
I bought last week a hyper-8 rolling chassis to convert brushless. Today on my first run my motor heat up to 195 (91c) only after 5 minutes. I use a medusa 36/60/2200, MMM and only 3s 5000mah 20c fligtmax lipo on it. lipo and esc was only around 90. I've only a 12T pinion that comes with the conversion set. I can easily turn the spur gear by hand when the motor isn't loaded and the gear mesh is OK. I've no time for opening the diffs right now.. Can this heat problem be based only on the little pinion or should I search for any problem on diffs?