Hi Minecraft

So last night i played minecraft with my friends, and friend 'A' was hosting so i was player 'B'
It went well
But when we wanted to continue today, we change pc's and i logged in on player A's pc, and player 'A' logged in on my pc

Though when starting the game, we both had player A's experience and items, which means that i, player 'b', had lost everything i had on me

Why does this happen, and can we do anything about it?

I am technically stupid, so please dumb it down for a person who is shit with computers

Best regards
Hobie Obarzanek