After you beat Lugia, head over to the Alpha ruins and beat the shadow Entei. When you do go back to New Bark Town and talk to your mom, she will want you to go and pick flowers for her in Cherrygrove City.
In the past - Cherrygrove City there is an old lady who talks about seeing Bellossoms. If you get a Bellossom then place it in the first place in your team so it shows as you walk then talk to the old lady she will give you some flowers. Go back to the present then go back to New Bark Town and talk to your mom, she will give you a Togepi.