30 weeks and irritated with toddler

I feel so very bad but my son is 3 years old. I’m currently 30 weeks pregnant and I just can’t handle all the rough house. He is constantly running towards me when I’m sitting on the couch. I’m constantly flinching or ducking away so he doesn’t hurt my belly. He is always pressing down on my belly. I’m very small to begin with so there’s no much cushion. I can feel the baby being pushed. We have tried to explain so many times that he can’t do this because there’s a baby in mommy’s belly but I don’t think he grasps it yet. But he is just so full of energy and I am slowly down and I feel terrible about. I start to get short and snap at him because I just can’t handle it. Does it get better once I give birth? I feel awful feeling this way about my own son.