When you said that you were applying for a PR card but you are already in Canada I am assuming that it is a PR card renewal?
Has your old card expired? If it hasn't you can still use your old card to travel.
If your old card has expired do you have any proof (such as a receipt for the renewal fee) that you have submitted your PR renewal application?
If not do you still have your COPR (the piece of paper you submitted to immigration the day you landed). That is proof of your landed immigration status.
If you don't have anything, you have to register at a visa office in the US (such as Buffalo) before you can re-enter Canada.
http://www.permanent-resident-card.com/freqaskedquestions.aspx (see #9)
