Lately my 3 1/2 month old DS will only nurse from 1 side and then when I offer the other breast he is not interested at all. Then at the next feeding I'll feed him from the side he didn't want and when I offer the other side he doesn't want it. He doesn't seem hungry after eating. He is eating every 2 hours right now and gaining weight fine. Is this normal that he is filling up on one side?