Please help! BFP lighter

Kay
Hi all! We got our BFP on the 31st with a digital. I tested with FRER and saw two lines, not incredibly strong but definitely visible. Tonight I just tested with FRER again and the line was visibly less dark than a day ago when I originally tested.
I did test around 11:30 pm (oh and I'm around 13 DPO) so maybe that affected it?! I'm really worried it's the beginning of a chemical pregnancy. 
Any help would be amazing