Is this an illegal defensive move"

I was playing 1 on 1 against a dude bigger than me, but not by much. I'm 6'2" 200 and he's probably 6'3" 220 ish. I was also lifting when he was lifting earlier at the rec center and he was definitely lifting heavier weight so he's stronger.

Anyway, he posted me up, and every time he would turn, I would block his movement with my shoulder, resulting in a very well contested shot or a block. He would never complain at all, but then again, playing 1 on 1, we're men and we let a lot of contact go unless it was a blatant foul. It was very effective though, but I'm wondering if I was fouling him in the process.

I ended up losing 9-11 because I eventually got winded and stopped going for 50-50 plays.

yep, that sounds like a foul in a reffed game .

but pick up game, you will probably get away with it depending on your opponet.