Negative exp is normal. It always happens when pokemon evolve, due to an increase in their exp base. Since their exp base increases, more experience is required to level up - Therefore, you have to train off the negative exp to catch up/adjust to the exp base, and to pay your debt for all of the levels you raised on a smaller exp base, which required less exp to level up, if that makes sense.
It's only a visual error that it appears negative, but train through the negative exp, and you'll be fine.