Canadian - US tax treaty

I am a US citizen. I lived in Canada for a few years while I worked in the US. Due to some issues with Canadian Revenue I did not pay income taxes on my salary (I did pay US taxes). When I was divorced we had to sell our house in Canada. At that time the house sale was frozen until I gave them all the information and filed taxes. I did this. the sale and profits from the sale were released. A year later I received a bill for $15K for back taxes. I had moved back to the US and was told that there is no way that I can be forced to pay these taxes. Now, 2 years later, I have a letter from the IRS saying that I owe Canada. It says that if I am a US citizen and can prove it, I may not owe the US but I would still owe Canada. The question...can the US IRS force me to pay the taxes to Canada? Or do I just continue on my way and never enter into Canada again? Thanks