is my cat playfighting or actually scared?

i often like to play fight with my cat, (shes 9 for context) but sometimes i feel like she doesn’t see it as playfighting? i don’t know if im overthinking this or not. When we playfight she’ll swat me with her paw and jump out to bite me (normal , yk) and i’ll also swat her with my hand back (gently! i promise) as we like exchange blows she’ll like back herself up, her pupils super wide and her ears back, but she looks really scared and i think i may just be overthinking it honestly, i mean throughout all of this she’s exposing her stomach to me. but i’m just worried i’m stressing her out?