I am visiting my brother here in the US, and I am staying for 3 weeks. I used esta in order to come to the US. We want to visit Montreal (Canada) for two days and come back to the US. (We are going there by car)

Will I have problem coming back to the US? I mean esta doesn't expire for at least 2 more months, but if I leave the states, and come back two days later, would esta still be valid? can I come back? Thanks!