Low Weight

I went to the doctor for an ultrasound today and left feeling a bit depressed and emotional. The ultrasound tech said at 31 weeks 6 days my son is 3lbs 11oz and is in the 17 percentile. She said he is a small baby which made me worry because the way she made it sound was as if he was going to be tiny and have a health issue. I literally feel like eat all day and everyday. I’ve gained 25lbs but my doctor told me I need to exercise. However, my feet have started swelling so I have to sit down and elevate them while I’m at work. I feel like I’ve done something wrong or something is wrong with me. Am I overreacting or overanalyzing things? Has anyone had a baby with low birth weight?