I have a Lakewood Cottager with 19' class A and 6' single wall stove pipe. The chimney is straight up and out. I got some kindeling going and added some pine siding scraps. Then after 10 minutes add some 1.5" x 1.5" white oak ( kiln dried) @ 8 pcs. Damper is 2/3 open and air also 2/3 open. I leave for 7-8 minutes to toss some lumber up to the second floor of the barn and then smell smoke. I go down and the stove is rumbling and then blows smoke out the air inlets and does it again 10 -15 seconds later. Negative pressure? What did I do wrong?