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ahhhhhh 😅
It's usually a sign of improper stance which affects your body structure and balance. The results is the energy from the hit travel through you horizontally sending you sideways. With a good structure (and other stuffs), you should be able to redirect that energy downwards to the ground.

My stances are apparently okay (sifu doesn't really fix them unless he's being super nitpicky and I'm a millimetre off or something), I'm just pissweak XD

Or you unconsciously break your stance/posture when seeing a hit coming or tense up. From my previous experience with Wing Chun, I've been told that even week people should be able to withstand a good amount of incoming energy and I could see that with my great-grand-master (Ip Chun) who is a tiny old man but his stance is super stable.

Anyway, the importance is to have fun and find some benefits in the practice.

Actually now that you mention it that would be exactly it as I'm usually moving when I get hit (and the only reason I get hit is because I either mistimed moving or realised I did the wrong thing and stopped because my automatic reaction is to get out of the way first and fix them up later which is not very helpful when you're supposed to be practising something specific XD)