My horse pulls on me when I lunge him

I own a 12 year old mini gelding and I am currently lunging him every other day to get him in shape for an upcoming show. Normally, he does very well and we have no problms but recently he has been pulling on me very hard. He only does it one way (clockwise) and I don't understand what started to make him do it. He will pull so hard on me that he actually bends his neck to the side. I only weigh 123 pounds while he weighs 256 so it's hard for me to keep hold of him. To make matters worse, I have a bad wrist and it's not good to have him pull on it like that. I can't afford a round pen or else I would get one. So far I have tried to pull back on him when he pulls but that just makes him stop. I have also tapped him with the whip when he does it but that just makes him pull harder. I'm at a loss as to why he started to do this and how to fix it. I know I'm not over working him because he doesn't even pant when I'm done. If you have any idea of how to make him stop, that would be great.

I have a mini gelding who likes to do the same thing. He will do this just out of simple rebellion when I lunge him with the halter. With the halter it doesn't bother him to pull on me. But what I have found with him that works is either a chain over then nose or put a bit in his mouth.

Put the chain through his halter around his nose, and lunge as normal. this way when he pulls the chain is a more direct and painful pressure, so he will avoid pulling on it.