Why does my horse kick while I try to get on her?

So my 16 year old Morgan is in perfect health (vet checked) and has just started to kick her left back leg if you even put your foot in the stirrup. She’s not in pain and all her tack fits perfectly. It started off with just her pinning her ears when I put the saddle pad and saddle on, she is a bit girthy and would... show more So my 16 year old Morgan is in perfect health (vet checked) and has just started to kick her left back leg if you even put your foot in the stirrup. She’s not in pain and all her tack fits perfectly. It started off with just her pinning her ears when I put the saddle pad and saddle on, she is a bit girthy and would pin her ears when oh get on. After you’re on her she’s perfectly fine her ears are forward, no kicking or bucking. She can do both English and western but prefers western. She will pin her ears while dismounting and sometimes would walk away and kick her leg but not it’s gotten worse and becoming a safety issue for other horses and riders. Help would be extremely helpful. :)
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