Afraid im not eating enough

I'm 29 weeks and I get full so easy now! I have a bowl of cereal and feel like I ate a 5 course meal. I'm afraid I'm not eating enough to help my baby gain weight. I can't tell how much she weighs until I get my next ultrasound and I have no idea when that will be. I've just been eating small meals and snacks. I haven't gained any weight yet either.