11dpo and a negative HPT but a very dark (almost positive) OPK . . .?

The OPK is darker than it was at 9dpo and quite close to a positive but the HPT says negative. If I was pregnant why is my OPK so strong when I know when I ovulated? And if it was picking up HCG then surely the HPT would say pregnant too? Not sure what to think, I have felt 'car sick' for the past two days and felt quite tired but that could be other things . . . argh! Opinions?? Experiences?? Thanks in advance.