Illegal defense?

[JoJo10]

I have a question about defending the low post that i'm hoping someone can help with.

I'm only 5'7" , but am a beast on the low post and boxing people out due to my low center of gravity. I've noticed that when most guys get tired of me backing into them on the low post, they put their knee in between my legs (from behind) and the top of their thigh under my butt (almost like using their leg as a chair). They also do this when I am boxing them out for a rebound. When this is done, it makes it virtually impossible for me to get any leverage and box out/ back into. Due to most people being taller than me, this technique lifts me off the ground and gives me no chance.

So, my question is, is this a legal maneuver? If not, what is the call?

Thank you in advance!