can a non compete I've signed in USA be held against me in Canada ?

Assuming that the non-compete is held to be enforceable there is no reason of which I am aware that it would not be enforced in Canada.  Although non-competes are more likely to be enforced the more narrowly they are drawn (i.e. a provision barring a former employee from competing for 4 months within a mile of his former employer is more likely to be enforced than one barring the employee from competing for 4 years anywhere within 200 miles) but, under certain circumstances, a worldwide non-compete might be enforced.