Could be as simple as a bathroom vent of a Kitchen vent running. Wind can cause downdraft in stovepipe.
Yep. Your problem is not necessarily a clogged stovepipe. I have had a problem similar to yours every now and then during the past 30 years. Usually, it has something to do with the weather, ie., a lot of wind or screwy barometric pressure or something like that...
BTW, after going 3 or 4 years paying a sweep to come ream out my stovepipe I finally bought my own kit at Home Depot. It's basically a nylon (or wire) brush and a bunch of skinny, pole pieces that screw together one at a time as you feed the brush down the pipe. The kits are not expensive at all. Now I scoot up on the roof a few times each winter and ream out the pipe myself. It's easy and free; I can hardly imagine how much money I have saved over the decades, and I can clean my stovepipe whenever I think it needs it.
Good luck.