Negative with afternoon test & BFP with FMU later?

Question

BFP with FMU later?" /> 
Has anyone tested negative in the afternoon at 11dpo then received a BFP later with FMU? Wondering if I should buy more tests... negative was with a first response. 
Thought I had implantation bleeding/ spotting about 5 days ago...
Af is due in 3 days.