Thats good news for you to hear, though it does surprise me somewhat that they were so blase about "oh well you don't know how long you are gpping to stay ........." Canada doesn't let anyone in on the basis of "I'm just gonna hang around and see what happens."
You need to remember you are applying as a visitor (which means you are not coming here to live), however when you get questioned at the border you need to be as truthful as you can.
Reiterate that you are absolutely going to go home at the end of your visa stay (although if you do get admitted you can absolutely apply to have the visa extended). And I would be totally upfront and honest about your immediate intentions. You are going to see your girlfriend with a view to taking the relationship further and discussing your future.
If you are asked about work, answer them truthfully - that right now you know absolutely that you have no rights to work, though should the relationship progress, then you will be looking further into employment visas and stress that you will do this from either the USA (if you have to go back) or in Canada if things are going well.
Advise that you wish to overstep no boundries, but want to try and do the best / right thing for all concerned - ie Immgration wise and personally.
Be polite and non threatening and honest.
And good luck
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