It looks like off-gassing. Hydrometer would be great, however since you already racked to secondary - 7 days is about as long as you want, time to get it moving off the hops. Off gassing could because of simple air temp change and the CO2 is being released. I would not consider these bubbles fermentation. When I dry hopped the other week, I saw the same and proceeded as my plan. All is well.