Jonin medal gives very little direct power (only about 100). The the guy is gone for 2 months and suddenly got a ton of power over you; he most likely bought power from other sources.

Stats from jonin medal is static. 60 hp, 30 attack, 30 nin; which is only about 100 pow; no more. You do not get any additional pow based on level.

You can gain a lot of power quickly from going through ultimate training and such; but a big power increase is definitely not because of jonin medal.
This post was last edited by IamAscrub at 2017-4-25 11:17

Either from using stocked up mats for things like battle armor/refines; or just plainly buying power. Just because someone say they didn't buy it doesn't mean they are not doing it. You can gain a fair chunk of power very quickly by spending coupons/ingots on charm packs; if you didn't finish ultimate training, for example.