It's hard to tell from the photo, but it might just be a clump of mucous. Your plug is still very delicate this early on, and pieces of it can come out after vigorous sex or exercise anyway. Try not to worry too much, it's only a small bit of mucous anyway! I see women in my clinic (I'm a maternity nurse) who worry about this sort of thing and it isn't an issue until later in pregnancy. Your vagina is still constantly producing large quantities of mucous and even if it is a bit of plug, it'll "grow" right back