OPK false smiley face? Confused...?

Day 10 of my cycle today and I got a smiley face on OPK at 2pm. Tried again at 6pm and got a circle...no smiley...I don't understand!? Can I get a false smiley face? Surely I haven't missed my chance already....didn't have sex lastnight (planned to tonight) and worried I've missed opportunity already. Tests say you're most fertile for nextr 24-48 hours....can anyone help???