Scupper

I've played until the 4th badge and my basicly only pokemon was Venusaur on LvL55 with Leftovers. However I was trading with a friend, because she wanted to evolve her kadabra so we traded. After the trade, somehow my Venusaur with Leftovers was gone and my Bulbasaur with an Oran Berry was back. I effectively lost 50 level. Is there a way i can get my Venusaur back? PS: I attached a screenshot of my starter which devolved