14 weeks 3 days weight gain has started

I've been holding at my normal 135-136 lbs but today I weighted myself and I'm 138.6! It seems like it was just overnight! I'm 5'8 with a long torso so I just have a small belly at this point. 
When did you start really gaining? I've been walking and swimming a fair amount so I'm really hoping to stay fit for an easier delivery and PP recovery. Gonna try prenatal yoga next week. 
I know I shouldn't be too worried about gaining a lot of weight but I really want a natural birth and my midwife said that staying in shape and not gaining more than 20-30 lbs will help make that possible. Stronger pelvic floor muscles means less tearing.