You can see your activity rate in the group tab if you hover over the "Activity Rate: ###" line to compare. Though it is far less convenient than just having it displayed next to where it says Last 3-days like it should be

For a group as active as yours though, that does seem far off. Those numbers are close to what my group has and we only have about 10-15 people who are active most of the time. It's strange it's only effecting two of the groups on your server though, not everyone. And to have such a sudden drop off in both groups I wouldn't consider it just a sudden lack of activity. It's normal for a few people to be busy this time of year and be less active, but enough to half the activity in two groups at the same time??