How do you differentiate baby movements vs. gas, growing pains, etc?

I swear I feel my baby moving, but my doctor says it’s probably too early. I thought I started feeling it at 16 weeks. Not kicks, just a slight shift in weight from one side to another. Now I’m at 21½ weeks and am not feeling painful kicks yet, but more like sudden specific movements. I think I’m not crazy and know the difference b/t gas and baby movements, but does everyone think this too at first? If so, how will I know when it’s the baby moving?

Oh yeah… and the sonogram tech said she could feel the baby kick through the little device she was rubbing on my stomach! (granted, she was pressing really hard)..... So I think I may be feeling the real thing.