Facebook: a rant

Notice none of the groups are listed
next to the groups I've actually joined

        Recently I've been scrolling through my newsfeed on Facebook and there are random posts from "groups" that I didn't know I had joined. This has happened about five times now. Each time I go to the group page and "leave the group." These groups are always titled something like "Top Health Tips" or "Health and Wellness."
        When it first started happening I noticed that one friend I had on Facebook was always a member of the group as well. I didn't actually know the person, but I accepted his friend request when I saw that he also went to Ohio Northern. After realizing he was the common denominator between me and each of these groups, I unfriended him. It's been about two weeks since I deleted him. But today I found another group hidden in my newsfeed! I'm tired of it and I don't know how I keep getting spammed like this.
         So friends, do you have any suggestions? Has this ever happened to you? Do you think it's because once upon a time I "liked" something about health and I became a target for these pages? Why do they only appear in my newsfeed, but not my groups list on the left side of the page?