Generally the open circle happens if you mark yourself as sleep deprived for that day.
If you mark "spotting" under Menses/Spotting, then you get the *. Maybe they think it looks like a spot?? LOL Seriously though, those are the things that I know for sure to cause that to happen. Someone else may be able to give you another reason for the open circle, but the * is definitely if you choose spotting for the day. Hope this helps!